.clang-format

rust: hooking into Python code

We introduce a new class called 'rustlazyancestors'
in the ancestors module, which is used only if
parsers.rustlazyancestors does exist.

The implementation of __contains__ stays unchanged,
but is now backed by the Rust iterator. It would
probably be a good candidate for further development,
though, as it is mostly looping, and duplicates the
'seen' set.

The Rust code could be further optimized, however it already
gives rise to performance improvements:

median timing from hg perfancestors:
- on pypy:
    before: 0.077566s
    after:  0.016676s -79%
- on mozilla central:
    before: 0.190037s
    after:  0.082225s -58%
- on a private repository (about one million revisions):
    before: 0.567085s
    after:  0.108816s -80%
- on another private repository (about 400 000 revisions):
    before: 1.440918s
    after:  0.290116s -80%

median timing for hg perfbranchmap base
- on pypy:
    before:  1.383413s
    after:   0.507993s -63%
- on mozilla central:
    before:  2.821940s
    after:   1.258902s -55%
- on a private repository (about one million revisions):
    before: 77.065076s
    after:   16.158475s -80%
- on another private repository (about 401 000 revisions):
    before:  7.835503s
    after:   3.545331s -54%

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