This is a set that is optimized to be space and time efficient for
small or large numbers of elements that implement the Copy trait.
The Copy constraint is convenient, but with more work this set could
work for arbitrary types (supporing Eq and Hash, of course).
Eventually it will need a better name.
To run the benchmark suite, cd into bench and then run
cargo +nightly run --release
This ought to work, provided you've got the nightly rust installed with rustup.