examples: Add example code for user C modules, both C and C++.

Add working example code to provide a starting point for users with files
that they can just copy, and include the modules in the coverage test to
verify the complete user C module build functionality.  The cexample module
uses the code originally found in cmodules.rst, which has been updated to
reflect this and partially rewritten with more complete information.

examples/usercmodule/cppexample/examplemodule.h

@@ -0,0 +1,5 @@
+// Include MicroPython API.
+#include "py/runtime.h"
+
+// Declare the function we'll make available in Python as cppexample.cppfunc().
+extern mp_obj_t cppfunc(mp_obj_t a_obj, mp_obj_t b_obj);