perf: Introduce TypeKey instead of String keys for named types

[frankdavid]

Overview
We can achieve serious performance improvements by introducing a new TypeKey struct instead of String for candid types represented by Type::Var. When deserializing a candid message, we need to store information about these types (e.g. in the TypeEnv struct). Previously, we used BTreeMap with String keys. This is slow and a significant amount of time is spent looking up types by name.

Optimization 1: distinguish indexed (integer) keys from String keys
When decoding type information from the message header in binary_parser.rs, the types are indexed. Previously these indexed types were assigned a String key by calling format!("table{index}"). Working with integer keys is much faster.

Optimization 2: use HashMap instead of BTreeMap
Looking up values in a HashMap is much faster (especially if the keys are small).

Optimization 3: precompute hash of String-based TypeKeys

Breaking changes
This is a breaking change:

• In Type::Var(var) var now has type TypeKey instead of String. Calling var.as_str() returns &str and var.to_string() returns a String. The string representation of indexed variables remains table{index} to maintain compatibility with previous versions.
• TypeEnv now contains a HashMap instead of BTreeMap. Code that relied on the iteration order of the map (e.g. env.0.iter()) should make use of the newly added TypeEnv::to_sorted_iter() method which returns types sorted by their keys.

[github-actions]

Name	Max Mem (Kb)	Encode	Decode
blob	4_224	20_459_233 ($\textcolor{green}{-0.00\%}$)	12_081_559 ($\textcolor{green}{-0.02\%}$)
btreemap	75_456	4_022_698_958 ($\textcolor{green}{-0.00\%}$)	15_635_820_900 ($\textcolor{green}{-0.22\%}$)
nns	192 ($\textcolor{red}{50.00\%}$)	1_862_487 ($\textcolor{green}{-18.07\%}$)	3_138_802 ($\textcolor{green}{-42.90\%}$)
nns_list_proposal	1_088	6_810_279 ($\textcolor{green}{-3.80\%}$)	60_219_418 ($\textcolor{green}{-26.46\%}$)
option_list	128	7_140_329 ($\textcolor{red}{0.00\%}$)	24_538_182 ($\textcolor{green}{-4.70\%}$)
text	6_336	20_455_518 ($\textcolor{red}{0.00\%}$)	17_839_609 ($\textcolor{red}{0.00\%}$)
variant_list	128	7_141_482 ($\textcolor{green}{-0.01\%}$)	23_301_701 ($\textcolor{green}{-4.68\%}$)
vec_int16	16_704	168_582_732 ($\textcolor{red}{0.00\%}$)	1_006_655_081 ($\textcolor{green}{-6.25\%}$)

• Parser cost: 18_056_685 ($\textcolor{green}{-3.89\%}$)
• Extra args: 2_347_729 ($\textcolor{green}{-27.54\%}$)

Click to see raw report

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Benchmark: blob
  total:
    instructions: 32.54 M (-0.01%) (change within noise threshold)
    heap_increase: 66 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 20.46 M (-0.00%) (change within noise threshold)
    heap_increase: 66 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 12.08 M (-0.02%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: text
  total:
    instructions: 38.30 M (0.00%) (change within noise threshold)
    heap_increase: 99 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 20.46 M (0.00%) (change within noise threshold)
    heap_increase: 66 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 17.84 M (0.00%) (change within noise threshold)
    heap_increase: 33 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: vec_int16
  total:
    instructions: 1.18 B (improved by 5.40%)
    heap_increase: 261 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 168.58 M (0.00%) (change within noise threshold)
    heap_increase: 261 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 1.01 B (improved by 6.25%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: btreemap
  total:
    instructions: 19.66 B (-0.18%) (change within noise threshold)
    heap_increase: 1179 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 4.02 B (-0.00%) (change within noise threshold)
    heap_increase: 159 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 15.64 B (-0.22%) (change within noise threshold)
    heap_increase: 1020 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: option_list
  total:
    instructions: 31.68 M (improved by 3.68%)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 7.14 M (0.00%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 24.54 M (improved by 4.70%)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: variant_list
  total:
    instructions: 30.45 M (improved by 3.63%)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 7.14 M (-0.01%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 23.30 M (improved by 4.68%)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: nns
  total:
    instructions: 23.88 M (improved by 12.83%)
    heap_increase: 3 pages (regressed by 50.00%)
    stable_memory_increase: 0 pages (no change)

  0. Parsing (scope):
    instructions: 18.06 M (improved by 3.89%)
    heap_increase: 3 pages (regressed by 50.00%)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 1.86 M (improved by 18.07%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 3.14 M (improved by 42.90%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: nns_list_proposal
  total:
    instructions: 67.03 M (improved by 24.66%)
    heap_increase: 17 pages (no change)
    stable_memory_increase: 0 pages (no change)

  1. Encoding (scope):
    instructions: 6.81 M (improved by 3.80%)
    heap_increase: 3 pages (no change)
    stable_memory_increase: 0 pages (no change)

  2. Decoding (scope):
    instructions: 60.22 M (improved by 26.46%)
    heap_increase: 14 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: extra_args
  total:
    instructions: 2.35 M (improved by 27.54%)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Successfully persisted results to canbench_results.yml