stream: Fix chunking sensitivity.

Fixes #1 (closed).

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

Benchmark change:

• Before: test gigabyte_stream ... bench: 146,492,395 ns/iter (+/- 3,035,585)
• After: test gigabyte_stream ... bench: 148,758,785 ns/iter (+/- 2,188,461)