Use SSE2 instructions for scrypt, if available

This is a simplified, faster version of the original SSE2 implementation of scrypt by Colin Percival. It should provide a ~50% speedup over the generic implementation. While not as fast as hand-written assembly, it is easier to maintain and more portable, and works for both x86 and x86-64 targets.
The decision whether to use the SSE2 version is made at compile time, based on the `__SSE2__` macro.