nested-control-flow

Test nested control flow:

    check(
        \\var x: i64 = 15;
        \\if (x > 10) {
        \\    if (x > 20) { 1 } else { 2 }
        \\} else {
        \\    3
        \\}
    , 2);

## Part 5: Functions

This is the biggest step. A function call has to: evaluate arguments, save the caller's variables, bind arguments to parameters, execute the body, capture the return value, and restore everything. It's a lot of state management. But the reward is fib(10) = 89.