While amd64 always supports the SSE2 instruction set extension,
other architectures don't (including 32 bit x86). For the platforms
that don't, we'll use the reference C implementation by default, but
allow users to override it with the BLAKE2_USE_SSE2 option.
The crypto "library" only consists of a single function at the
moment because that's all that users have asked for so far. It is
also highly experimental. We will make an attempt to keep the public
interface (i.e. the interface provided by Library_Crypto.c) stable,
but it might still change if necessary. The internal interface
(provided via the global _Crypto proplist) is not for public
consumption and will probably change at some point.