Added tests to "Interfacing with external C code" part 1

Added tests to "Implementing the buffer protocol" part 1

See #2362.

Enhance "wrapping C libraries".

Added tests to "Early binding for speed" part 1

Extended the examples of string.rst and put them in the examples directory for testing.

Added tests to "Extension types" part 1

Move a comment from the documentation into the example that it refers to, in order to avoid inconsistencies (e.g. line number references) and also to make it visible to those who only look at the file.
See #2353.

Added tests for "memory allocation" Part 1

Fix C compiler warnings about an unused error label and an unused helper function if only builtin types are used and no external extension types.

Generate short wrappers for "__await__()" special method to adapt its signature for the PyCFunction entry in PyMethodDef.
See #2363.

Generate short wrappers for special methods like "__next__()" to adapt their signature for the PyCFunction entry in PyMethodDef.
Previously, their cast to a two-argument PyCFunction was incorrect.
See #2363.

Ignore exact function pointer type in PyCFunction pointer comparison since there is no use in figuring out the correct cast here.

Reduce Cython's module size of "Actions", "Scanners", "StringIOTree", "Visitor" and "Code" by disabling auto-pickling.

Fix function signature casts in PyMethodDef structs that lead to warnings in gcc-8 (and actually make use of the warnings in gcc-8).

Update cpp examples

Added tests to "profiling tutorial" part 5

Added tests to "profiling tutorial" part 4

Added tests to "profiling tutorial" part 3

Added tests to "profiling tutorial" part 2

Changed xrange for range while moving a piece of code from the profiling tutorial to the examples directory.