kdf v0.1.0

[tarcieri]

Initial release

[tarcieri]

This is wired up to hkdf and all of the PBKDFs in https://github.com/RustCrypto/password-hashes

It hasn't been implemented for anything other than hkdf in https://github.com/rustcrypto/kdfs because we'll need to define per-algorithm ZSTs to do the trait impl on.

There isn't much to it but it's useful functionality, particularly for PAKEs where there isn't otherwise a good trait to hook into the low-level KDF interface.

If there's one thing left to discuss it's probably the Error type: the crate defines its own unit struct, but some other options are an associated Error type, or an enum Error with some variants for common KDF errors.

Edit: let's try the simple error type and see if anyone requests something different.