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.