Proposal: Miri-checked tests for the unsafe data structures #126

The LinkedList bug from #124 was found only by Miri: Python tests and ASan never reach that code path. Miri can check hashbrown, LinkedList and LazyHeap, since they don't call into libpython. But right now there are only 7 Rust tests, LazyHeap has none, and CI doesn't run them.

What I'd suggest:

1. Add unit tests for LazyHeap and LinkedList (11 tests, including a random-operations test against a simple model for each).
2. Add a CI workflow: cargo test on stable plus cargo +nightly miri test.

I already have both in my fork. All 18 tests pass on current main, natively and under Miri. The jobs take ~30 s and ~1–1.5 min and run in parallel with Python Test, so PRs won't wait longer. With the pre-#124 linked_list.rs both jobs fail (run).

Would you be interested in a PR with these?