Add rustc_hash to benchmarks

[smmalis37]

https://crates.io/crates/rustc-hash

I'd be curious to see how things compare. Their docs say they outperform FNV, which is promising.

[tkaitchuck]

rustc-hash is basically the same as fxHash just with different constants. You can see it in the graph.
Based on this issue: rust-lang/rustc-hash#18
I have been toying around with an improved version of rustc-hash. When that is ready I will post a PR there and update aHash's fallback with anything I learn.

Also: "Faster than FNV" isn't saying much, as you can see from the graph here: https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md#speed
FNV is one of the slowest popular hashing algorithms.