py/runtime: Check that keys in dicts passed as ** args are strings.

Prior to this patch the code would crash if a key in a ** dict was anything
other than a str or qstr.  This is because mp_setup_code_state() assumes
that keys in kwargs are qstrs (for efficiency).

Thanks to @jepler for finding the bug.

py/obj.h

@@ -720,6 +720,7 @@ qstr mp_obj_str_get_qstr(mp_obj_t self_in); // use this if you will anyway conve
 const char *mp_obj_str_get_str(mp_obj_t self_in); // use this only if you need the string to be null terminated
 const char *mp_obj_str_get_data(mp_obj_t self_in, size_t *len);
 mp_obj_t mp_obj_str_intern(mp_obj_t str);
+mp_obj_t mp_obj_str_intern_checked(mp_obj_t obj);
 void mp_str_print_quoted(const mp_print_t *print, const byte *str_data, size_t str_len, bool is_bytes);
 
 #if MICROPY_PY_BUILTINS_FLOAT