impl trait for "const generics"

[xunilrj]

Description

This PR is part of #6860.
It implements "impl traits" for "const generics", which means syntax such as:

trait A {
    fn my_len(self) -> u64;
}

impl<T, const N: u64> A for [T; N] {
    fn my_len(self) -> u64 {
        N
    }
}

This also opens the space for simplifying our current implementation of AbiEncode for arrays. Today we implement for each size, which limits us to a specific size. With this one can have just one "impl item", and support arrays of arbitrary size.

Both implementations will coexist in codec.sw until we stabilize "const generics".

#[cfg(experimental_const_generics = true)]
impl<T, const N: u64> AbiEncode for [T; N] 
where
    T: AbiEncode,
{
    fn abi_encode(self, buffer: Buffer) -> Buffer {
        ....
    }
}

#[cfg(experimental_const_generics = false)]
impl<T> AbiEncode for [T; 0]
where
    T: AbiEncode,
{
    fn abi_encode(self, buffer: Buffer) -> Buffer {
        ...
    }
}

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6946 will not alter performance

_{Comparing xunilrj/impl-trait-const-generic (ba56ca8) with master (8a3d8df)}

Summary

✅ 22 untouched benchmarks

[jjcnn]

A few minor issues and corrections, but overall it looks very good.

[IGI-111]

One nit but otherwise LGTM