rust: Remove use of `py.eval()`
The previous Rust code allocated an intermediate `Vec`, converted that
to a Python list, then used `eval` to run Python code that converts that
list to a Python set.
rust-cpython exposes Rust bindings for Python sets, let’s use that instead
to construct a set directly.
Differential Revision: https://phab.mercurial-scm.org/D10328
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).