Convert macros to static inline functions to avoid macro pitfalls,
like duplication of side effects:

* DK_ENTRIES()
* DK_UNICODE_ENTRIES()
* PyCode_GetNumFree()
* PyFloat_AS_DOUBLE()
* PyInstanceMethod_GET_FUNCTION()
* PyMemoryView_GET_BASE()
* PyMemoryView_GET_BUFFER()
* PyMethod_GET_FUNCTION()
* PyMethod_GET_SELF()
* PySet_GET_SIZE()
* _PyHeapType_GET_MEMBERS()

Changes:

* Add also assertions in some functions using "CAST" macro to check
  the type of the arguments when Python is built with assertions
  (debug build).
* Remove an outdated comment in unicodeobject.h.
* PyCode_GetNumFree() doesn't cast its argument: the replaced macro
  already required the exact type PyCodeObject*.
* PyCode_GetNumFree() casts PyCode_GetNumFree.co_nfreevars from int
  to Py_ssize_t to be future proof, and because Py_ssize_t is
  commonly used in the C API.