.clang-format

rust: rustlazyancestors.__contains__

This changeset provides a Rust implementation of
the iteration performed by lazyancestor.__contains__

It has the advantage over the Python iteration to use
the 'seen' set encapsuled into the dedicated iterator (self._containsiter),
rather than storing emitted items in another set (self._containsseen),
and hence should reduce the memory footprint.

Also, there's no need to convert intermediate emitted revisions back into
Python integers.

At this point, it would be tempting to implement the whole lazyancestor object
in Rust, but that would lead to more C wrapping code (two objects) for
little expected benefits.

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