Check variable initialization

[tiif]

In today's formality, we never check if a variable is initialized before use, so the test below passed although it shouldn't.

crate Foo {
    fn foo () -> u32 = minirust() -> x {
        let x: u32;
        exists {
            bb0: {
                statements {
                }
                return;
            }
        }
    };
}

The test above is equivalent to

fn foo() -> u32{
    let x: u32;
    return x;
}

[xonx4l]

@tiif I think we're missing a check for uninitialized variables or maybe a comparator that compares whether the final block variables exist in initial path.