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.

rust: translated random test of missingancestors This is a Rust implementation of the random DAG generator and related incrementalmissingancestors tests against a naive brute force implementation. It is provided as an integration test, so that it won't run by default if any unit test fails. In case of a failed example, all needed information for reproduction is included in the panic message, (this is how `test_remove_ancestors_from_case1()` has been generated), as well as the random seed. The whole test is rerunnable by passing the random seed in the TEST_RANDOM_SEED environment variable. The other parameters (numbers of iterations) can be passed in the TEST_MISSING_ANCESTORS environment variable. An alternative would have been to expose to Python MissingAncestors<VecGraphs> but that would have meant pollution of the release build used from Python, whereas we do it in this changeset within the tests submodule Differential Revision: https://phab.mercurial-scm.org/D5417