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.

Relevant options/directives must go into the cache fingerprint #2237.

If the test runs too fast, the timestamps will be the same
despite overriting it.

Use dict versioning in Py3.6+ to make the lookup of Python module globals almost as fast as C globals.

I made the section that drops options quite verbose because it took me a while to
figure out whether an option should be included or not. I hope I got everything
right.

Allow access to long/int internals (and other builtin types).

CYTHON_CACHE_DIR should also set the compiler cache