remove unnecessary Result-wrapping in `crypto.rs`

[cosmicexplorer]

Broken out from #287.

1. Separate CTR and CBC encryption methods in ctr and cbc submodules.
2. Define pub const sizes for array lengths.
3. Require fixed-size arrays to en/decryption methods to remove EncryptionError and one case of DecryptionError.
4. Rename methods to aes_256_* and aes_256_ctr_hmac_* for consistency.

TODO

• [ ] Figure out how to apply this comment: https://github.com/signalapp/libsignal/pull/287#discussion_r647031224.

[jrose-signal]

These aren't pub for a reason: if someone wants to use these operations, they should get them directly from the RustCrypto crates. These are pretty much just convenience wrappers because of how they're used within the protocol. (The hardcoded "first ten bytes as a MAC" is something that a generalized library probably wouldn't hardcode.)

I think you're right about EncryptionError, though. Someone has to check that these keys are the right length, but it can happen a level up. And then I think DecryptionError collapses down to a struct instead of a one-case enum (not that it matters so much for an implementation detail).

[cosmicexplorer]

These are pretty much just convenience wrappers because of how they're used within the protocol.

Ok, so in light of this I've changed it to pub(crate) mod crypto; in lib.rs, removed the doctests, and re-focused this PR on just removing unnecessary Results by enforcing fixed-size slices where possible.

However, on top of that, I've also kept in:

• the cbc and ctr module organization,
• some docstrings for parts that I thought weren't immediately clear,
• new en/decryption test cases using Rng::gen() (vs the existing test cases which only check a single literal hex key).

I can revert any of that as desired.

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