Interpreter panics on unreachable code due to return

[Hugo Wangler]

When the interpreter gets an input containing a return statement followed by other statements it panics. This shouldn't happen since a program containing unreachable code should still be able to run.

ex:

fn test() -> i32 {
    return true;
    if (true) {
        return false;
    }
}

Can be solved by adding a next field to return. This part of the function will never be interpreted but it won't panic.