rust-index: add fast-path for getting a list of all heads as nodes This avoids a lot of back-and-forth between Python and Rust. We forgo adding a fast-path in the `filteredchangelog` case yet. If it shows up in profiling, we might add the variant with a filter.

rust-index-cpython: cache the heads' PyList representation This is the same optimization that the C index does, we just have more separation of the Python and native sides.

rust-index: use a `BitVec` instead of plain `Vec` for heads computation The `Vec` method uses one byte per revision, this uses 1 per 8 revisions, which improves our memory footprint. For large graphs (10+ millions), this can make a measurable difference server-side. I have seen no measurable impact on execution speed.

rust-index: implement faster retain heads using a vec instead of a hashset This is the same optimization that the C index does, we're only catching up now because this showed up as slow in benchmarking.

rust-index: allow inlining VCSGraph parents across crates

rust-index: allow inlining `parents` across crates

rust-index: allow inlining `check_revision` across crates

rust-index: document safety invariants being upheld for every `unsafe` block We've added a lot of `unsafe` code that shares Rust structs with Python. While this is unfortunate, it is also unavoidable, so let's at least systematically explain why each call to `unsafe` is sound. If any of the unsafe code ends up being wrong (because everyone screws up at some point), this change at least continues the unspoken rule of always explaining the need for `unsafe`, so we at least get a chance to think.

rust-index: renamed `MixedIndex` as `Index` It is simply not mixed any more, hence the name had become a future source of confusion.

rust-index: stop instantiating a C Index The only missing piece was the `cache` to be returned from `revlog.parse_index_v1_mixed`, and it really seems that it is essentially repetition of the input, if `inline` is `True`. Not worth a Rust implementation (C implementation is probably there for historical reasons).