Extremely slow due to the underlying SHA implementation

[AntonMeep]

Because this seems to be extremely slow compared, to say, OpenSSL. While clearly it would benefit from optimizations similar to what fastpbkdf2 uses, biggest bottleneck seems to be GNAT.SHA* modules. Ideally, I would prefer to have a separate crate for sha and hmac that would implement these algorithms in a platform-independent, portable and hopefully faster manner.

Example of a simple benchmark on 2^10 iterations:

Implementation	Time (s)
PBKDF2	5.527577500
OpenSSL	1.494409900

[AntonMeep]

Since v2.0.0 this crate now uses other implementation of SHA and HMAC algorithms. These are still slow, however.