std.digest

[Márcio Martins]

Hi!

To my surprise, std.digest.MurmurHash3 doesn't work in CTFE.

Would it be hard to have it explicit in the documentation if a 
particular Phobos symbol works in CTFE? Maybe it could be manual, 
and vote-based, to avoid building infrastructure around it.

Also, MurmurHash3 only outputs 32-bit or 128-bit hashes, which 
leaves us without any modern fast 64-bit digest. Would anyone 
oppose pulling an XXHash implementation into Phobos?