Block hash algorithms now return an array.

Trying to simplify the block hash algorithms so that they look mostly
like algorithms, less like streams. I've removed the use of buffer
unpacking functions in Murmur3.32, pretty much unwinding the call into
the loop body.

Block functions now return an array of 32-bit words for the hash key.
The caller determines what to do with the words. Somewhere down the
road, this library might support interfaces in addition to
`crypto.Hash`.

Closes #29.