rust-index: using the `hg::index::Index` in ancestors iterator and lazy set Since there is no Rust implementation for REVLOGV2/CHANGELOGv2, we declare them to be incompatible with Rust, hence indexes in these formats will use the implementations from Python `mercurial.ancestor`. If this is an unacceptable performance hit for current users of these formats, we can later on add Rust implementations based on the C index for them or implement these formats for the Rust indexes. Among the challenges that we had to meet, we wanted to avoid taking the GIL each time the inner (vcsgraph) iterator has to call the parents function. This would probably still be acceptable in terms of performance with `AncestorsIterator`, but not with `LazyAncestors` nor for the upcoming change in `MissingAncestors`. Hence we enclose the reference to the index in a `PySharedRef`, leading to more rigourous checking of mutations, which does pass now that there no logically immutable methods of `hg::index::Index` that take a mutable reference as input.

revlog: always use a Rust index for REVLOGv1 if rustext is present We are about to change classes such as `rustext.AncestorsIterator` to take a Rust index, hence we cannot have the option not to use the Rust index. Note: this can be refined depending on whether we want to keep this option or not. We will have to make two versions of `AncestorsIterator` and its sibling to support REVLOGV2 and CHANGELOGv2 anyway. Meanwhile, this is the simplest change to make the tests pass.

rust-index: disabling flagprocessor tests The list of flags supported by the Rust index is not dynamic, hence flagprocessor has no chance to work.

rust-index: support `unionrepo`'s compressed length hack Explanations inline.

rust-index: honour incoming using_general_delta in `deltachain` It looks to be a leftover from some past, but the C index considers only the value passed from Python whereas up to now the Rust index was using the value of its attribute. As a middle ground, we make this argument of `deltachain` optional from the Python side, with the Rust implementation only defaulting to its attribute. This way, we reduce false leads when a difference in results is spotted.

rust-index: use interior mutability in head revs and caches For upcoming changes in `hg-cpython` switching to the `hg-core` index in ancestors iterators, we will need to avoid excessive mutability, restricting the use of mutable references on `hg::index::Index` to methods that actually logically mutate it, whereas the maintenance of caches such as `head_revs` clearly does not. We illustrate that immediately by switching to immutable borrows in the corresponding methods of `hg-cpython::MixedIndex`

rust-index: add Sync bound to all relevant mmap-derived values All readonly mmaps are Sync as far as Rust is concerned. Integrity of the mmap'ed file is a concern separate to Rust's memory model, since it requires out-of-program handling via locks, etc. This will help when we start sharing the Rust Index with Python.