Check pattern refutability the same way exhaustiveness is checked
Provide a witness pattern for non-exhaustive patterns

Fixed #4321
Add support for fixed size vectors in let/arg patterns

Fixes #7784
Add unreachability detection for missized patterns of fixed size vectors

Fixed #13482
Fix #14393

String patterns should have a single constructor of arity 0.
Address review comments
auto merge of #14731 : jakub-/rust/pattern-matching-refactor, r=nikomatsakis

This PR is changing the error messages for non-exhaustive pattern matching to include a more accurate witness, i.e. a pattern that is not covered by any of the ones provided by the user. Example:

```rust
fn main() {
	match (true, (Some("foo"), [true, true]), Some(42u)) {
		(false, _, _) => (),
		(true, (None, [true, _]), None) => (),
		(true, (None, [false, _]), Some(1u)) => ()
	}
}
```

```sh
/tmp/witness.rs:2:2: 6:3 error: non-exhaustive patterns: (true, (core::option::Some(_), _), _) not covered
/tmp/witness.rs:2 	match (true, (Some("foo"), [true, true]), Some(42u)) {
/tmp/witness.rs:3 		(false, _, _) => (),
/tmp/witness.rs:4 		(true, (None, [true, _]), None) => (),
/tmp/witness.rs:5 		(true, (None, [false, _]), Some(1u)) => ()
/tmp/witness.rs:6 	}
```

As part of that, I refactored some of the relevant code and carried over the changes to fixed vectors from the previous PR.

I'm putting it out there for now but the tests will be red.