Move crates into crates/ and rename plural modules to singular

[PhilippGackstatter]

Move crates into crates/ and rename plural modules to singular.

Closes #1021 and #1062.

I renamed plurals to singular whenever the containing type was singular but the module was not. For example Asset was in assets/mod.rs, so I renamed it to asset/mod.rs.

However, when the containing type is plural or we have or anticipate multiple types to be contained in a module, I left them as plural. Modules deliberately left as plural:

• crates/miden-objects/src/constants.rs which contains multiple constants.
• crates/miden-objects/src/transaction/tx_args.rs which contains TransactionArgs.
• crates/miden-objects/src/note/details.rs which contains NoteDetails.
• crates/miden-objects/src/transaction/inputs.rs which contains InputNotes, ToInputNoteCommitments, TransactionInputs.
• crates/miden-objects/src/note/inputs.rs which contains NoteInputs.
• crates/miden-objects/src/transaction/outputs.rs which contains OutputNotes, TransactionOutputs.
  • This one also contains OutputNote, so it's impossible to satisfy the above criteria here.
• errors modules, since they usually contain multiple error types.
• crates/miden-objects/src/transaction/events.rs module which contains multiple event constants.
• crates/miden-objects/src/note/assets.rs which contains NoteAssets.
• crates/miden-tx/src/auth/signatures which might contain more signatures in the future.
• I also left crates/miden-lib/asm intact, because we anticipate to have multiple kernels, account_components, note_scripts, etc. so using plural seems fine. For the same reason the modules in crates/miden-lib/src/account/ were left as-is.

Feel free to disagree, though, I don't have a strong opinion on those. This is just the rationale I've followed.

[bobbinth]

All looks good! Thank you!