feat: allow cancelling the solving process

[aochagavia]

Introduces a should_cancel_with_value method in DependencyProvider,
that returns an Option<Box<dyn Any>>. This method is called at the
beginning of each unit propagation round and before potentially blocking
operations (like [Self::get_dependencies] and [Self::get_candidates]).

If should_cancel_with_value returns Some(...),
propagation is interrupted and the solver returns
Err(UnsolvableOrCancelled::Cancelled(...)), bubbling up the value to
the library user (this way you can handle any additional details
surrounding the cancellation outside of the solver).

Closes #8.

Remarks

• SolverCache::provider is now pub(crate). Is that all right?
• The current design requires implementors of DependencyProvider to use interior mutability to track cancellation. I think that's reasonable, because otherwise we would need to take &mut self in all of DependencyProvider methods, but I'm mentioning it here in case anyone has a different opinion.