incremental murmur hash

Reviewer: @jnealtowns

With this API, the caller doesn't need to have all the data in one big continuous buffer. Instead, it can be hashed as it's read from disk or a socket.

The usecase I have in mind is the IVS tcam module, which needs to mask and hash packet keys. It's faster to fuse those loops together instead of producing a temporary masked buffer and then hashing that.