Use `kdf` traits for PBKDFs

[tarcieri]

Right now spake2 and srp suggest they should be used in conjunction with PBKDFs, but do not make it easy to do so or provide ready-made recipes for using them in conjunction with a PBKDF.

aucpace uses the password-hash crate to achieve this, then extracts the output from the computed password hash.

Instead, all of our PAKE implementations could use forthcoming traits from the kdf crate to abstract over PBKDF algorithms (i.e. that we have implemented in the https://github.com/RustCrypto/password-hashes repo)

See: https://github.com/RustCrypto/traits/pull/1879