Stream implementation sensitive to chunking

The StreamHasher struct in stream.rs is sensitive to to the chunking of data.

E.g. Hashing [u64::MAX, 0] is different to hashing [u64::MAX] then [0].

I have added a test to demonstrate in my fork at https://gitlab.redox-os.org/pantsman0/seahash/blob/stream-tests/src/stream.rs#L132

The output is:

---- stream::tests::chunked_equiv stdout ----
thread 'stream::tests::chunked_equiv' panicked at 'assertion failed: `(left == right)`
  left: `7214075340457006594`,
 right: `13128668274129567273`', src/stream.rs:141:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.


failures:
    stream::tests::chunked_equiv