Passing a Heap& to it only to then call interpreter() on that is weird.
Let's just give it the Interpreter& directly, like some of the other
to_something() functions.
This patch teaches UpdateExpression how to use a Reference. Some other
changes were necessary to keep tests working:
A Reference can now also refer to a local or global variable. This is
not fully aligned with the spec since we don't have a Record concept.
