Add control flow

So far our language can parse and evaluate floating point expressions.
However each expression and it's subexpressions always gets evaluated. To
do more complex computations, we want to be able to control what gets
evaluated and what doesn't.

This is known as control flow, and the two most famous imperative
constructs are probably the if statement and for loop. We will add them
to our language in this chapter, and finally put our boolean expressions
(42 < i) to good use.