Cleanups for SHA2 in preparation of the next Python PR

[msprotz]

This is basically replicating some of the work that I did for sha2-224/256 onto sha2-384/512:

• add a copy function
• write spec equivalence proofs so as to enable code sharing between sha2-384 and sha2-512

This is all in preparation for python/cpython#101707

[github-actions]

[CI] Important!
The code in dist/gcc-compatible, dist/msvc-compatible, and dist/wasm
is tested on cryspen/hacl-packages.
dist is not automatically re-generated, be sure to do it manually.
(A fresh snapshot can also be downloaded from CI.)
Then check the following tests before merging this PR.
Always check the latest run, it corresponds to the most recent version of this branch.
All jobs are expected to be successful.
In some cases manual intervention is needed. Please ping the hacl-packages maintainers.

• Build, Test, Benchmark: Build on Linux (x86, x64), Windows (x86, x64), MacOS (x64, arm64), s390x, Android (arm64) and test on Linux (x86, x64), Windows (x86, x64), MacOS (x64).
• Performance Regression Tests: Navigate to the terminal output in “Run benchmarks”. The comparison with the main branch will be at the bottom. The run fails if the performance regresses too much.
• OCaml bindings: Build & Tests
• JS bindings: Build & Tests
• Rust bindings: Build & Tests

[msprotz]

Sadly, there's a lot of whitespace changes because of the latest krml changes

[R1kM]

Thanks!

[polubelova]

Thanks!