Allow tests to override the default closure impls

[JakobDegen]

In tests, closures can now have explicit Fn* impls, in which case the default impls will no longer be emitted for that closure. These types of impls are restricted to being non-generic over the type. This allows reproducing #688 in a chalk test.

The mechanism for this is an absolute hack; if there's any impl Fn* for MyClosure then the parser will generate a custom LocalImplAllowed clause which is later searched for during WF checking of impls and before emitting default closure impls for that type. The upside of this is that we can re-use the plumbing on the chalk-solve side, especially the WF checking. This probably breaks coherence checking for Fn* traits in tests, but that doesn't seem like an issue.

[bors]

:umbrella: The latest upstream changes (presumably #753) made this pull request unmergeable. Please resolve the merge conflicts.