Implement new hashing

[ironcev]

Description

This PR implements new hashing, as explained in the tracking issue #7256. New hashing is an opt-in experimental feature with the feature flag new_hashing.

The new hashing hashes lengths as prefix to the content hash for the following built-in and std types:

• string slices (str)
• arrays ([T; N])
• std::vec::Vec<T>
• std::bytes::Bytes

Additionally, the PR adds missing Hash implementations, "old", without lengths, and new for:

• string arrays (str[N])
• raw slices (raw_slice)

Some of the std types that use the above types internally for storing their content still hash only the content, without the length prefix of the container. The reason for this is that:

1. the choice of the container is just an implementation detail, and
2. the semantics of the type requires only the content (mostly of the fixed, type-specific length) to be hashed.

A typical example would be std::b512::B512 where, same to b256 we want to hash only the content.

Notice that this does not cause the security issue described in #7256.

Here is the complete list of those types:

• std::crypto::secp256k1::Secp256k1
• std::crypto::secp256r1::Secp256r1
• std::crypto::message::Message
• std::crypto::public_key::PublicKey
• std::b512::B512

Some of the implementations differ based on the combination of const_generics and new_hashing features. Therefore, on CI, we have a step testing both experimental features together.

Additionally, the PR:

• adds testing of const_generics experimental feature to CI.

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.

[zees-dev]

Nice; LGTM 👍