Mutual recursion does not work

[nielstron]

Describe the bug
Mutual recursion does not work in opshin. Concretely, the following code fails to compile

def even(n: int) -> bool:
    if n == 0:
        return True
    else:
        return odd(n - 1)
        
def odd(n: int) -> bool:
    if n == 0:
        return False
    else:
        return even(n - 1)


def validator(a: int) -> int:
    a = 1
    a = "hello"
    return a

This could be solved on the type level by first extract function signatures and placing them all in scope. A seperate question is whether the compiler can handle this (are the later-defined functions in scope?)