poly1305: Fixups based on ChaCha20Poly1305 experience #16
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Add a function for inputting data into Poly1305 which is padded to its block size, a.k.a. the `pad16()` function from RFC 8439. This is useful for implementing authenticated encryption constructions using Salsa20 family ciphers (i.e. Salsa20Poly1305, ChaCha20Poly1305) which is the main use case for Poly1305 to begin with.
This outsources constant time comparisons to `MacResult`, so downstream crates don't need to rely on additional crates to perform them beyond what `MacResult` is already using.
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This makes some API changes based on my experience using this crate to implement the ChaCha20Poly1305 AEAD:
RustCrypto/AEADs#3
Poly1305::input_padded()function which pads the input to Poly1305's block size: this is used in several places for both Salsa20Poly1305 and ChaCha20Poly1305 (26c8c12)MacResult- Poly1305's main use cases are effectively as a MAC and the main thing you want to do with a tag is a constant time comparison, so leveraging MacResult for this makes sense (98cf577)