transfered checked type casts

Transfered overriding in extension types.

Transfered warning from the reference guide to the user guide.

Improved "Types" section.

Transfered cdef statement.

Transfered declaring data types.

Reducing the overhead here is actually helpful since many KeyErrors will never be instantiated.

Transfered the Cython file types in the userguide.

Speed up the recursion to children in Visitor.py by avoiding cpdef call overhead (most transforms are Python classes with class dicts that require an override check).

Simplify some method calling code by providing a (better) fallback implementation for "__Pyx_PyObject_GetMethod()".

Fixed wrong info in the readme.

Clarified cython's capabilities about C99's VLA.

Fold bound method unpacking into method lookup to 1) reduce code duplication and 2) at least avoid keeping the method object around any longer than necessary if we couldn't avoid creating it in the first place.

Improved the readme.

Made the rendering of the CONTRIBUTING file pretty.

Closes #2229.

Add size_type and difference_type to std::list and add initial unittests specific for std::list.