Add Keypair::from_bytes()

[json420]

I'd like to add a Keypair::from_bytes() method modeled after ed25519_dalek::SigningKey::from_bytes():

https://docs.rs/ed25519-dalek/latest/ed25519_dalek/struct.SigningKey.html#method.from_bytes

This seems to be pretty standard API for most Public Key crypto libraries. Very useful for Know Answer Tests at the protocol level. Can be handy when integrating with specialized random number hardware. And in my particular case, I really need to provide the entropy because I'm doing application level entropy accumulation at each signature (and switching to a new keypair at each signature):

https://github.com/zebrafactory/zebrachain

I've tested this on Pop!_OS against Rust nightly, beta, and stable. It's also been tested via my GitHub test runs for ZebraChain against Rust stable on Ubuntu, MacOS, and Windows.

Thanks for your great pure Rust implementations of Dilithium and sphincsplus!

Have an awesome day!

[json420]

Oh, sorry about the noise in Cargo.toml. The change was needed to get it working on recent cargo versions.