rust-index: find_gca_candidates bit sets genericization This allows to use arbitratry size of inputs in `find_gca_candidates()`. We're genericizing so that the common case of up to 63 inputs can be treated with the efficient implementation backed by `u64`. Some complications with the borrow checker came, because arbitrary sized bit sets will not be `Copy`, hence mutating them keeps a mut ref on the `seen` vector. This is solved by some cloning, most of which can be avoided, preferably in a follow-up after proof that this works (hence after exposition to Python layer). As far as performance is concerned, calling `clone()` on a `Copy` object (good case when number of revs is less than 64) should end up just doing a copy, according to this excerpt of the `Clone` trait documentation: Types that are Copy should have a trivial implementation of Clone. More formally: if T: Copy, x: T, and y: &T, then let x = y.clone(); is equivalent to let x = *y;. Manual implementations should be careful to uphold this invariant; however, unsafe code must not rely on it to ensure memory safety. We kept the general structure, hence why there are some double negations. This also could be made nicer in a follow-up. The `NonStaticPoisonableBitSet` is included to ensure that the `PoisonableBitSet` trait is general enough (had to correct `vec_of_empty()` for instance). Moving the genericization one level to encompass the `seen` vector and not its elements would be better for performance, if worth it.

rust-index: core impl for find_gca_candidates and find_deepest This still follows closely the C original and not able to treat more than 63 input revisions (bitset backed by `u64` and one bit reserved for poisoning).

rust-index: add support for `reachableroots2` Exposition in `hg-cpython` done in regular impl block, again for rustfmt support etc.

hg-cpython: rev_pyiter_collect_or_else It will be useful to give callers the control on the generated errors

rust-index: add support for `computephasesmapsets` Exposition in `hg-cpython` done in the regular `impl` block to enjoy rustfmt and clearer compilartion errors.

rust-index: slicechunktodensity returns Rust result Ready for removal of the scaffolding. This time, we allow ourselves a minor optimization: we avoid allocating for each chunk. Instead, we reuse the same vector, and perform at most one allocation per chunk. The `PyList` constructor will copy the buffer anyway.

rust-index: add support for `_slicechunktodensity`

rust-index: headrevsfiltered() returning Rust result

rust-index: add support for `headrevsfiltered` The implementation is merged with that of `headrevs` also to make sure that caches are up to date.

rust-index: implement headrevs