Some changes in class hierarchies etc.; transforms no longer has a common
base class and VisitorTransform is a subclass of TreeVisitor rather than
the reverse. Also removed visitor use of get_child_accessors;
child_attrs is accessed directly (because of claims of overengineering :-) ).

--HG--
rename : Cython/Compiler/Transform.py => Cython/Compiler/Visitor.py

See the documentation of each class for details.

It is a rather big commit, however seperating it is non-trivial. The tests
for all of these features all rely on using each other, so there's a
circular dependency in the tests and I wanted to commit the tests and
features at the same time. (However, the non-test-code does not have a circular
dependency.)