Since integers are first class objects, we need to use an integer object
as the return address. The actual address can not be stored in an
instance variable since it is not an object.
The address is unique to the label and never changes after positioning
(using the int is next up)
Before creating DynamicJump, the FunctionCall got a register for a
possible jump address. Now that is handled by DynamicJump and
FunctionCall just needs the method, from which it determines the
binaryCode address
