.arcconfig

rust-cpython: management of shared libray suffix

Before this changeset, the shared library objects suffixes
were both (rustc output and Python input) hardcoded to '.so',
which is wrong for Python3 and non Linux targets.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}