Ability to specify optional return type

[duckinator]

(Opening as requested by @kayila after talking to her about it.)

While not restricting the return type is a reasonable default behavior for a scripting language, it would be nice to be able to opt-in to it when wanted.

I'm thinking something like this, where the -> int specifies the return type and is optional:

function factorial(int n) -> int {
    if (n == 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

If no return type is specified, it'd probably make the most sense to not restrict it in any way.