rust: pure Rust lazyancestors iterator This is the first of a patch series aiming to provide an alternative implementation in the Rust programming language of the _lazyancestorsiter from the ancestor module. This iterator has been brought to our attention by the people at Octobus, as a potential good candidate for incremental "oxydation" (rewriting in Rust), because it has shown performance issues lately and it merely deals with ints (revision numbers) obtained by calling the index, whih should be directly callable from Rust code, being itself implemented as a C extension. The idea behind this series is to provide a minimal example of Rust code collaborating with existing C and Python code. To open the way to gradually rewriting more of Mercurial's Python code in Rust, without being forced to pay a large initial cost of rewriting the existing fast core into Rust. This patch does not introduce any bindings to other Mercurial code yet. Instead, it introduces the necessary abstractions to address the problem independently, and unit-test it. Since this is the first use of Rust as a Python module within Mercurial, the hg-core crate gets created within this patch. See its Cargo.toml for more details. Someone with a rustc/cargo installation may chdir into rust/hg-core and run the tests by issuing: cargo test --lib The algorithm is a bit simplified (see details in docstrings), and at its simplest becomes rather trivial, showcasing that Rust has batteries included too: BinaryHeap, the Rust analog of Python's heapq does actually all the work. The implementation can be further optimized and probably be made more idiomatic Rust.