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/objstr.c

@@ -2062,6 +2062,12 @@ mp_obj_t mp_obj_str_intern(mp_obj_t str) {
     return mp_obj_new_str_via_qstr((const char*)data, len);
 }
 
+mp_obj_t mp_obj_str_intern_checked(mp_obj_t obj) {
+    size_t len;
+    const char *data = mp_obj_str_get_data(obj, &len);
+    return mp_obj_new_str_via_qstr((const char*)data, len);
+}
+
 mp_obj_t mp_obj_new_bytes(const byte* data, size_t len) {
     return mp_obj_new_str_copy(&mp_type_bytes, data, len);
 }