Refactor: binary-sv2::no-serde-sv2::codec crate

[rrybarczyk]

Background

This task is an outcome of the protocols Rust docs issues tracked in #845.

While documenting protocols::v2::binary-sv2::no-serde-sv2::codec in #1009, areas of potential code debt were identified. This issue servers as a place to list out these items to then be addressed in an organized manner. The initial Rust documentation effort was an immediate action, while a refactoring (which implies breaking API changes) is not so urgent priority, so for now we should leave this in the backlog for an appropriate moment in the future.

Identified Potential Code Debt

TODO

[Shourya742]

Remove unused trait Variable

pub trait Variable {
    const HEADER_SIZE: usize;
    //const ELEMENT_SIZE: usize;
    const MAX_SIZE: usize;

    fn inner_size(&self) -> usize;

    // That could be [u8; Self::HEADER_SIZE] but rust do not allow it
    fn get_header(&self) -> Vec<u8>;
}

[Shourya742]

#1231 (comment)