Python bindings to the Rust rpds crate.

What's here is quite minimal (in transparency, it was written initially to
support replacing pyrsistent in the referencing library). If you see something
missing (which is very likely), a PR is definitely welcome to add it.

Methods in general are named similarly to their rpds counterparts (rather than
pyrsistent's conventions, though probably a full drop-in pyrsistent-compatible
wrapper module is a good addition at some point).