ABI generate native integer(s) if fits

[maoueh]

Right now, all Ethereum Solidity types either Signer or Unsigned of any size are serialized to substreams::scalar::BigInt. This hinder a performance hit because arithmetic on pure native types would be much faster.

The goal of this task is to change the type generated in Rust for solidity Signed/Unsigned integers so they are aligned to the closest native type that support it.

For example:

• int8 -> i8
• int16 -> i16
• int24 -> i32
• ...
• (same with unsigned).

If bit size > 64, then we would render BigInt as before.

This will be a major breaking change as anyone with such ABI will generate new code using primitive types so it will drastically change their usage.

Maybe we should think about providing a backward compatibility option to the macro and codegen tool always_bigint=true (something like that) so that an easier update path is available.