nested-control-flow

Test nested control flow:

    testCase(
        \\var x: i64 = 15;
        \\if (x > 10) {
        \\    if (x > 20) { 1 } else { 2 }
        \\} else {
        \\    3
        \\}
    , 2, &varNames, &varValues, &varCount);

## 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 -- enough that passing five pointers around will finally break. We'll introduce a Context struct to bundle them, then keep going.

The reward: fib(10) = 89.