feat: Switch from AES-GCM to XChaCha20-Poly1305

[matheus23]

Summary

This PR

• Switches out the aes-gcm lib for chacha20poly1305
• Switched the Aes256Gcm struct for XChaCha20Poly1305
• Switched the nonces from aes::Nonce to XNonce
• Renamed AesError into CryptError (We still use AES-KW, so a more generic name made sense)
• Removed aes.rs file with AesKey struct
• Moved some custom key wrap/unwrap code from PrivateRef::to_serializable and PrivateRef::from_serializable to just use TemporalKey::key_wrap_{encrypt,decrypt}

The motivation for switching is simply the extended nonce. It let's us always generate a random nonce for every ciphertext and sleep well at night.
The canonical downside of not using AES-based ciphers is performance in case AES native instructions (AESNI) are available. However, this is not our main bottleneck & we'll run in lots of environments (e.g. Wasm) that don't benefit from AESNI.

[expede]

[codecov]

Codecov Report

Merging #305 (e4ba8f9) into main (7026a37) will decrease coverage by 0.04%.
The diff coverage is 69.69%.

❗ Current head e4ba8f9 differs from pull request most recent head 6383542. Consider uploading reports for the commit 6383542 to get more accurate results

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
- Coverage   58.31%   58.28%   -0.04%     
==========================================
  Files          43       42       -1     
  Lines        3097     3073      -24     
  Branches      744      740       -4     
==========================================
- Hits         1806     1791      -15     
+ Misses        800      793       -7     
+ Partials      491      489       -2     

Impacted Files	Coverage Δ
wnfs/src/private/encrypted.rs	66.66% <ø> (ø)
wnfs/src/private/forest/traits.rs	100.00% <ø> (ø)
wnfs/src/private/node/header.rs	76.92% <ø> (ø)
wnfs/src/private/keys/privateref.rs	56.66% <25.00%> (-0.10%)	⬇️
wnfs/src/private/node/keys.rs	82.05% <72.00%> (+7.05%)	⬆️
wnfs/src/private/file.rs	78.69% <100.00%> (ø)

[expede]

🙌

Huzzah! LGTM!