stream: Fix chunking sensitivity.

Closes #1

See merge request redox-os/seahash!2

This is a breaking change that will cause the major version to be
bumped.

- Rewrite `SeaHasher` to use its own inner state instead of the one in
`buffer.rs`
- Add tests for chunking for stream hashing
- Add tests for ensuring the same result for streaming and block hashing
- Add benchmark for stream hashing
- Change `...` to `..=` in `buffer.rs`

Fixes #4.

- Fix warning about docstrings in function body.
- Fix `helper::read_u64` on 32-bit systems.
- Fix `buffer::pop` to rotate in the correct direction.

Fix `buffer::pop`

See merge request redox-os/seahash!1

This fixes tests that fail when built on 32-bit systems.

The `pop()` function rotated the internal state in the wrong order, causing `pop()`s to overwrite `b`, `c` and `d` to be equal every time.
This has been fixed, and the `mut a` has been removed from `push()` too.