Refactor Signatures and Hash

[just-erray]

This PR

• Removes unnecessary code
• Adds comments explaining the function of each function
• Shortens the viable parts
• Cleans up and refactors code for clarity
• Adds and edits some functions for future use

in signatures folder and the blake3_u4.rs file.
(It doesn't cause any incompatibilities with the active work.)

[lucidLuckylee]

Hi @just-erray, is the current plan to use the blake3_u4.rs primitives in blake3.rs eventually (to switch to the u4 version)? Wondering, because this PR removes the hashing from blake3_u4.rs but doesn't integrate its primitives into blake3.rs yet.

[just-erray]

Hello, I believe the only BLAKE3 version we plan to use is the u4 version with the compact representation (which is supposed to be the case with the new chunker work, though @manishbista28 knows more about it). Since the compact version's hashing function serves the same purpose as the blake3_u4.rs's hashing function, I thought that it was redundant and removed it. I think the current plan is to remove all BLAKE3-related code except for blake3_u4_compact.rs and blake3_u4.rs, as the compact version depends on its functions. (Edit: just saw this issue, I think the removal of StackTracker might be messy but is definitely possible, nevertheless the current blake3_u4.rs works without any dependencies to other files so it is possible to remove blake3.rs all altogether. This PR refactors the code independently of the issue.)

[lucidLuckylee]

Cool thanks @just-erray!