non-copying (`*_of_slice`) variants of 128 bit murmur hashes

[eamsden]

#19 is an implementation of 32-bit mumur3 that does not require read() invocations or copying. The same implementation approach will work for the 128 bit versions of murmur3 (x64 and x86). This is a work issue to implement, bench, and test those variants.

@stusmall feel free to assign to me