Make TreeFragment.py more readable; copy substitution nodes and copy over pos attributes on substitutions

'#define PyString_Type PyBytes_Type' in Py3: 'str' is considered a builtin and internally mapped to PyString_Type, which must also work in Py3

Casting a non-Python pointer type to a Python type no longer
generates an incref, unless one is required for other reasons.
    [Arc Riley]

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

invalidate the cache of all subtypes when updating a type's tp_dict (algorithm copied from typeobject.c in Py3)