py/warning: Support categories for warnings.

Python defines warnings as belonging to categories, where category is a
warning type (descending from exception type). This is useful, as e.g.
allows to disable warnings selectively and provide user-defined warning
types.  So, implement this in MicroPython, except that categories are
represented just with strings.  However, enough hooks are left to implement
categories differently per-port (e.g. as types), without need to patch each
and every usage.

py/vm.c

@@ -1116,7 +1116,7 @@ unwind_return:
                     mp_uint_t unum = *ip;
                     mp_obj_t obj;
                     if (unum == 2) {
-                        mp_warning("exception chaining not supported");
+                        mp_warning(NULL, "exception chaining not supported");
                         // ignore (pop) "from" argument
                         sp--;
                     }