Add rayon feature flag and implement IntoParIterator

[crclark]

This seems to solve my issue #47 asking for a faster way to iterate. In my use case, I'm building up a large map (in parallel), then I need to write the entire thing to parquet files as fast as possible, so consuming the map across threads works great.

• Add a rayon feature flag to Cargo.toml.
• Add IntoParIterator and par_iter implementations for HashSet and HashMap.
• Add tests to show that the implementations don't lose any entries.

[ibraheemdev]

Just a heads up that I'm quite busy at the moment so it might take me some time to get to this. FWIW you can do this today using par_bridge with an owned guard (see #2).