Optimize gas for AssetId::new() and AssetId::default() functions

[bitzoic]

Description

The following changes remove the use of the std::hash::sha256 function in place of assembly to hash the ContractId and SubId of an asset. As the values hashed are of static length, we can perform the hashing in assembly avoiding the conversion to Bytes and concatenation of the two values done in sha256()

These changes result in a 93% reduction in gas usage.

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).
• 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.

[Braqzen]

I question the addition of asm back into the std. We want it to be readable but at the same time we want it to be cheap.
Consolidation via a single function wrapper is also better for future changes as we only need to change it in 1 place.