Enhance unexpected keyword argument detection

Before, incorrect use when calling a core function would just say
"extra keyword arguments given"; now, it will name the argument:

```python
>>> Synthesizer(bad_kwarg="boo")
TypeError: unexpected keyword argument 'bad_kwarg'
```

py/argcheck.c

@@ -133,7 +133,28 @@ void mp_arg_parse_all(size_t n_pos, const mp_obj_t *pos, mp_map_t *kws, size_t n
         #if MICROPY_ERROR_REPORTING <= MICROPY_ERROR_REPORTING_TERSE
         mp_arg_error_terse_mismatch();
         #else
-        // TODO better error message
+        #if CIRCUITPY_FULL_BUILD
+        mp_map_elem_t *elem = kws->table;
+        size_t alloc = kws->alloc;
+        for (size_t i = 0; i < alloc; i++) {
+            mp_obj_t key = elem[i].key;
+            if (key == MP_OBJ_NULL) {
+                continue;
+            }
+            bool seen = false;
+            for (size_t j = n_pos; j < n_allowed; j++) {
+                if (mp_obj_equal(MP_OBJ_NEW_QSTR(allowed[j].qst), key)) {
+                    seen = true;
+                    break;
+                }
+            }
+            if (!seen) {
+                mp_raise_msg_varg(&mp_type_TypeError,
+                    MP_ERROR_TEXT("unexpected keyword argument '%q'"), mp_obj_str_get_qstr(key));
+            }
+        }
+        #endif
+        // (for the !FULL_BUILD case, and as a fallthrough for the FULL_BUILD case, even though it SHOULD be unreachable in that case)
         mp_raise_TypeError(MP_ERROR_TEXT("extra keyword arguments given"));
         #endif
     }