Make chalk_solve::Solver UnwindSafe

[matklad]

In rust-analyzer, we use panics to implement cancellation. That means that, while we use chalk to solve some goal, the callbacks from chalk into rust-analyzer might panic, unwinding past the Solver.

This unfortunately leaves the solver in an inconsistent state at the moment. See https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/Logic.20panics for some more details.

This issue has been assigned to @marmeladema via this comment.

[matklad]

Note that we now try to catch panics within the chalk itself, to make rust-analyzer more robust:

rust-lang/rust-analyzer#2818

At the moment, we just reset chalk's state on panic, but it would be more convenient if chalk always was consistent after a panic.

[jackh726]

With #338 merged, closing this only requires some regression tests for panic safety. This is pretty easy, so I'll mark it as such.

[marmeladema]

@rustbot claim