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

This fixes #1399.

This fixes #1393.

so far only the inline cache looked at that environment variable

Use dict versioning in Py3.6+ to reduce the overhead of checking for the "C-line in tracebacks" runtime option.

Closes #1771.

Bug fix for boolean return using statement with internal commas.