Move VariableDeclarationGenerator to use a Collection instead of VariableCollection.

By making the interface of VariableDeclarationGenerator use VariableCollection,
it becomes very hard to figure out of a particular language needs all variables
(both input and output) generated.  This makes testing all these classes less
practical (the point of the interface).  By moving this logic out of a
generator, higher level logic for the language can decide how to use the
generator, without impacting the tests.