Removing files that don't verify or aren't used from `code`

[hacl-star]

Removing files that don't verify or aren't used from `code`

[s-zanella]

Just created a code/attic directory and moved a few stale files in code there. I identified them by going through files in code (except code/experimental) that weren't verified during CI.

• [Moved to attic/bignum] code/bignum/Hacl.Bignum.Lemmas.fst
• [Moved to attic/hash] code/hash/Hacl.Hash.SHA2_256.PrePost.fst
• [Moved to attic/salsa-family] code/salsa-family/AEAD.Chacha20.fst
• [Moved to attic/salsa-family] code/salsa-family/Hacl.Spec.Symmetric.Chacha20.fst
• [Moved to attic/ed25519] code/ed25519/Hacl.BignumQ.fst
• [Moved to attic/ed25519] code/ed25519/Hacl.Impl.Ed25519.Ladder.BigLoop.fst
• [Moved to attic/api] code/api/Hacl.Hash.fst
• [Removed, same as in code/api] code/api/interfaces/Hacl.Constants.fst
• [Moved to attic/lib] code/lib/Utils.fst
• [Moved to attic/lib/kremlin] code/lib/kremlin/Hacl.SBuffer.fst

I kept code/api/Hacl.SecretBox.fst, which looks stale and currently doesn't verify. It doesn't seem hard to fix.

I'm unsure about the role of code/api/interfaces. The fsti files in there don't seem to be used anywhere and, for instance, typechecking code/api/Hacl.Box.fst against code/api/interfaces/Hacl.Box.fsti fails.

I patched and added to CI some files meant to be verified that weren't, either because they regressed or were (un)intentionally omitted. Notably, the whole code/lib and code/lib/kremlin directories.

[beurdouche]

Directory attic was removed in the dev branch