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.c

@@ -0,0 +1,25 @@
+#include <examplemodule.h>
+
+// Define a Python reference to the function we'll make available.
+// See example.cpp for the definition.
+STATIC MP_DEFINE_CONST_FUN_OBJ_2(cppfunc_obj, cppfunc);
+
+// Define all properties of the module.
+// Table entries are key/value pairs of the attribute name (a string)
+// and the MicroPython object reference.
+// All identifiers and strings are written as MP_QSTR_xxx and will be
+// optimized to word-sized integers by the build system (interned strings).
+STATIC const mp_rom_map_elem_t cppexample_module_globals_table[] = {
+    { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_cppexample) },
+    { MP_ROM_QSTR(MP_QSTR_cppfunc), MP_ROM_PTR(&cppfunc_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(cppexample_module_globals, cppexample_module_globals_table);
+
+// Define module object.
+const mp_obj_module_t cppexample_user_cmodule = {
+    .base = { &mp_type_module },
+    .globals = (mp_obj_dict_t *)&cppexample_module_globals,
+};
+
+// Register the module to make it available in Python.
+MP_REGISTER_MODULE(MP_QSTR_cppexample, cppexample_user_cmodule, MODULE_CPPEXAMPLE_ENABLED);