RMC should have its own panic hook

[avanhatt]

Currently, when RMC panics, we panic with the Rustc hook that includes something like this:

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

We can use panic::set_hook to set our own custom hook that links to this repo instead.