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/example.cpp

@@ -0,0 +1,17 @@
+extern "C" {
+#include <examplemodule.h>
+
+// Here we implement the function using C++ code, but since it's
+// declaration has to be compatible with C everything goes in extern "C" scope.
+mp_obj_t cppfunc(mp_obj_t a_obj, mp_obj_t b_obj) {
+    // Prove we have (at least) C++11 features.
+    const auto a = mp_obj_get_int(a_obj);
+    const auto b = mp_obj_get_int(b_obj);
+    const auto sum = [&]() {
+        return mp_obj_new_int(a + b);
+    } ();
+    // Prove we're being scanned for QSTRs.
+    mp_obj_t tup[] = {sum, MP_ROM_QSTR(MP_QSTR_hellocpp)};
+    return mp_obj_new_tuple(2, tup);
+}
+}