rust-cpython: binding for headrevs() This uses the core `dagops::retain_heads` to give a Rust implementation to `mercurial.dagop.headrevs`. Testing happens for now from `test-rust-ancestors.py` (for quick and minimal change), but it'd made more sense to put the binary index data elsewhere and to create a new test python module

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{

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    "conduit_uri": "https://phab.mercurial-scm.org/api",

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    "repository.callsign": "HG",

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    "arc.land.onto.default": "@",

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    "base": "hg:.^"

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}