TentHash Rust: prep for v1.1.0 release.

Prep for TentHash-rs 1.0.

Tenthash Rust: documentation improvements.

Clean up documentation, etc. of the Rust implementation.

Switch back to xor-and-mix again instead of add-and-mix.

After consideration, there aren't any real benefits to using add
instead of xor here.  So we might as well use the simpler operation.

Work on documentation and API of the Rust crate.

Add benchmarks and clean up test vector tests.