Wasm: Add the core of initial WebAssembly language module support.

This adds the core of runtime WebAssembly[0] support. Future commits
will enable this in the Unit core and expose the configuration.

This introduces a new src/wasm directory for storing this source.

We are initially using Wasmtime[0] as the WebAssembly runtime, however
this has been designed with the ability to use different runtimes in
mind.

src/wasm/nxt_wasm.[ch] is the main interface to Unit.

src/wasm/nxt_rt_wasmtime.c is the Wasmtime runtime support. This is
nicely insulated from any knowledge of internal Unit workings.

Wasmtime is what loads and runs the Wasm modules.

The Wasm modules can export functions Wasmtime can call and Wasmtime can
export functions that the module can call.

We make use of both. The terminology used is that function exports are
what the Wasm module exports and function imports are what the Wasm
runtime exports to the module.

We currently have four function imports (functions exported by the
runtime to be called by the Wasm module).

1) nxt_wasm_get_init_mem_size

This allows Wasm modules to get the size of the initially allocated
shared memory. This is the size allocated at Unit startup and what the
Wasm modules can assume they have access to (in reality this shared
memory will likely be larger).

The amount of memory allocated at startup is NXT_WASM_MEM_SIZE which as
of this commit is 32MiB.

We do actually allocate NXT_WASM_MEM_SIZE + NXT_WASM_PAGE_SIZE at
startup which is an extra 64KiB (the smallest allocation unit), this is
to allow room for the response structure and so module developers can
just assume they have the full 32MiB for their actual response.

2) nxt_wasm_send_headers

This allows WASM modules to send their headers.

3) nxt_wasm_send_response

This allows WASM modules to send their response.

4) nxt_wasm_response_end

This allows WASM modules to inform Unit they have finished sending their
response. This calls nxt_unit_request_done()

Then there are currently up to eight functions that a module can export.
Three of which are required. These function can be named anything. I'll
use the Unit configuration names to refer to them

1) request_handler

The main driving function. This may be called multiple times for a
single HTTP request if the request is larger than the shared memory.

2) malloc_handler

Used to allocate a chunk of memory at language module startup. This
memory is allocated from the WASM modules address space and is what is
sued for communicating between the WASM module (the guest) and Unit (the
host).

3) free_handler

Used to free the memory from above at language module shutdown.

Then there are the following optional handlers

1) module_init_handler

If set, called at language module startup.

2) module_end_handler

If set, called at language module shutdown.

3) request_init_handler

If set, called at the start of request. Called only once per HTTP
request.

4) request_end_handler

If set, called once all of a request has been sent to the WASM module.

5) response_end_handler

If set, called at the end of a request, once the WASM module has sent
all its headers and data.

32bits

We currently support 32bit WASM modules, I.e wasm32-wasi. Newer version
of clang, 13+[2], do seem to have support for wasm64 as a target (which
uses a LP64 model). However it's not entirely clear if the WASI SDK
fully supports[3] this and by extension WASI libc/wasi-sysroot.

64bit support is something than can be explored more thoroughly in the
future.

As such in structures that are used to communicate between the host and
guest we use 32bit ints. Even when a single byte might be enough. This
is to avoid issues with structure layout differences between a 64bit
host and 32bit guest (I.e WASM module) and the need for various bits of
structure padding depending on host architecture. Instead everything is
4-byte aligned.

[0]: <https://webassembly.org/>
[1]: <https://wasmtime.dev/>
[2]: <https://reviews.llvm.org/rG670944fb20b226fc22fa993ab521125f9adbd30a>
[3]: <WebAssembly/wasi-sdk#185>

Signed-off-by: Andrew Clayton <[email protected]>