Change semantics of `return` to be an early return

Right now `return` is a keyword but only used to embed expressions into statements. This is fine for a core representation like System C, but not appropriate for source.

That is, right now
```
def foo() = {
val x = if (true) return 42 else 0;
x + 1
}
```
currently returns `43` and not `42` (which might be confusing to people).