test partial decryption

[jowparks]

Decrypting notes runtime, decrypting the first 32 bytes vs whole note:

tl;dr:
decrypt note for spender ~ 40% reduction in time
Full note vs first 32 bytes: 300 us vs 173 us

decrypt note for owner ~75% reduction in time
Full note vs first 32 bytes: 270 us vs 73 us

Test:
To just decrypt first 32 bytes in rust instead of full note we can change:
let note = Note::from_spender_encrypted(transmission_key, &shared_key, &self.encrypted_note).unwrap();
to

let mut truncated_encrypted_text = [0u8; 32];
truncated_encrypted_text.copy_from_slice(&self.encrypted_note[..32]);
let partial: [u8; 32] = aead::decrypt_partial(&shared_key, &truncated_encrypted_text.to_vec()).try_into().unwrap();
jubjub::Fr::from_bytes(&partial).unwrap();

[jowparks]

Done to avoid having to modify return types

[jowparks]

This seek value needs to change based on the position of the plaintext (for example if you want to decrypt bytes 32-64 instead of 0-32, this would need to seek to 96. Hardcoded here for test