Contract ABI Specification Format

[sezna]

If this issue would be better on the HLL, let me know. I believe this is a good place for it though, as it impacts the VM, the chain, and the compiler.

In the most technical sense, what format do we want to use to specify the ABI? In the compiler I have a mapping of public entry points to ASM, but I'm uncertain what serialization format would be the best.

1. I know we mentioned using hash-based indexing, but once those are specified, what output (from the compiler) would be easiest for other tooling to ingest?
2. What is "standard" for ABI description formats in our space?

[adlerjohn]

While the ABI does only matter to higher-level constructs than the VM, using ABI encoding in transaction data very much affects other parts of the tech stack, including developer-facing SDKs. As such, it might not be such a bad idea to include a specification of the ABI format in this repo.

For reference, the Solidity ABI specification can be found here: https://docs.soliditylang.org/en/v0.7.6/abi-spec.html. Ideally we should follow a similar format, of specifying the function selector, followed by specifying how things are encoded recursively. We only have a dozen or so types, so the rules should be fairly short FuelLabs/sway#4.

solc outputs a JSON file of a contract's ABI: https://docs.soliditylang.org/en/v0.7.6/abi-spec.html#json. We could do something similar, maybe using a better file format.