rust-cpython: binding for headrevs() This uses the core `dagops::retain_heads` to give a Rust implementation to `mercurial.dagop.headrevs`. Testing happens for now from `test-rust-ancestors.py` (for quick and minimal change), but it'd made more sense to put the binary index data elsewhere and to create a new test python module

rust-cpython: moved py_set() utility to conversion module We're still hoping to get rid of it eventually, but we're going to need it from outside the `ancestors` module before that.