Abort control flow upon a return, RET, or RVRT.

[sezna]

This should unblock the final hard blocker for swayswap.

What does this do?

In Rust, if a branch aborts control flow, then its return type doesn't matter. Here is an example.

We want this behavior so we can implement .unwrap() -- panicking if there is no value present in an Option requires that we don't type check the else branch in case of a deterministic abort.

How does this do?

This PR introduces a .deterministically_aborts() method on AST nodes, expressions, and code blocks which returns a true if, in its control flow, there is a RVRT, RET, or return that is impossible to avoid. We then special case some type checking in branched code around that, as well as take it into consideration when type checking expressions against their annotations.

Other things

While implementing this, I found a bug in return statement type checking -- we were not type checking returns that are on the RHS of a variable declaration. For example, the below compiles when it should not.

fn main() -> u64 {
  let x = return true;
}

This PR includes a fix for that.

Closes #308

[otrho]

LGTM otherwise.

[adlerjohn]

Merge conflicts. Could you move the testcase into the directory structure of #965?

I've tested locally and can confirm that old-compiler does not compile the testcase and new-compiler does.

[adlerjohn]

LGTM, will defer to the actual compiler devs.