Can now call functions, though only to execute their code -- arguments
and return values aren't implemented yet (probably going to need some
kind of call stack for that).  Also due to complications if end was
called within a function, I got rid of the return value for statements,
which previously indicated whether the end statement had been called.
Now instead there's a flag in GeppettoProgram.  (I never liked that
statements had a return value anyway.)