rust-index: implement common_ancestors_heads() and ancestors() The only differences betwwen `common_ancestors_heads()` and `find_gca_candidates()` seems to be that: - the former accepts "overlapping" input revisions (meaning with duplicates). - limitation to 24 inputs (in the C code), that we translate to using the arbitrary size bit sets in the Rust code because we cannot bail to Python. Given that the input is expected to be small in most cases, we take the heavy handed approach of going through a HashSet and wait for perfomance assessment In case this is used via `hg-cpython`, we can anyway absorb the overhead by having `commonancestorheads` build a vector of unique values directly, and introduce a thin wrapper over `find_gca_candidates`, to take care of bit set type dispatching only. As far as `ancestors` is concerneed, this is just chaining `common_ancestors_heads()` with `find_deepest_revs`.

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.