Add unaligned version of indexWordArray#

indexWordArray# takes an offset in whole words, making it impossible to do unaligned reads on platforms that support such reads. This has performance implications for some programs e.g. implementations of !MurmurHash.

I propose we add versions of indexWord*Array that take byte offsets. The user is responsible for only using those on platforms that support them and the implementation is free to crash if they're used elsewhere.