Implemented starknet_getClassAt and starknet_getClassHashAt

[Trantorian1]

Pull Request type

• Bugfix

What is the current behavior?

• get_class_hash_at has been marked as TODO and does not return.
• get_class_at has been marked as TODO and does not retrieve the correct contract ABI.
• Contract ABI has incorrect values in ABI function output field.
• Failed blockifier to starknet-core conversion on contrat classes which did not define an L1_HANDLER or CONSTRUCTOR in their entrypoints.

Resolves: #32 #33 #34

What is the new behavior?

• Contract ABI can now be retrieved based on contract address, not just class hash.
• starknet_getClassHashAt retrieves the correct class hash.
• starknet_getClassAt retrieves the correct class and ABI.
• ABI function output field now has the correct values (was previously using the value of ABI function input field).
• blockifier to starknet-core contract class conversion now works even if L1_HANDLER or CONSTRUCTOR is not defined.
• Added info log output for when a class is being downloaded.

Does this introduce a breaking change?

No changes have been made to class or ABI storage, so there is no need to synchronize the database

Other information

⚠️ Note that due to how the genesis block is currently being stored, contract address to class equivalences relations are not being inserted for block 0. This causes issues for starknet_getClassHashAt and starknet_getClassAt on any class/contract defined on block 0. See #54 for more details.

starknet_getClass, starknet_getClassHashAt and starknet_getClassAt were briefly tested on the following classes:

• 0x010455c752b86932ce552f2b0fe81a880746649b9aee7e0d842bf3f52378f9f8
• 0x07595b4f7d50010ceb00230d8b5656e3c3dd201b6df35d805d3f2988c69a1432
• 0x071c3c99f5cf76fc19945d4b8b7d34c7c5528f22730d56192b50c6bbfd338a64
• 0x07543f8eb21f10b1827a495084697a519274ac9c1a1fbf931bac40133a6b9c15
• 0x074a7ed7f1236225600f355efe70812129658c82c295ff0f8307b3fad4bf09a9

More tests will need to follow in Ditto.

⚠️ Note that there still appears to be a deserialization issue at block 20731. This is likely to be similar to #116.