Early std.crypto

[Piotr Szturmaj]

https://github.com/pszturmaj/phobos/tree/master/std/crypto

This is some early work on std.crypto proposal. Currently only MD5, HMAC 
and all SHA family functions (excluding SHA0 which is very old, broken 
and no longer in use). I plan to add other crypto primitives later.

I know about one SHA1 pull request optimized for SSSE3. I think native 
code must be there to support other non x86 CPUs and SIMD optimization 
may be added at any time later.

Any opinions are welcome. Especially if such design is good or bad, and 
what needs to be changed.

Thanks :)