This rewrites the rust implementation to use many new features of the
current version of rust.
The refactor is much more concise (only 2/3rds the size) and switches
to using a lot of the more functional features (iterators, closures,
etc) that have been added or improved in rust.
Unfortunately, the implementation is a fair bit slower (about 30% on
perf3). It's not clear why this is the case but concision and being
more idiomatic wins over performance.
