rust: box ConfigValueParseError to avoid large result types
clippy emits a warning that all the Result types are way too large
because of HgError includes ConfigValueParseError as one of the variants,
so its size is 136 bytes.
By boxing ConfigValueParseError we're hopefully making everything faster
"for free".
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}