(en/decode_coding_object): Fix `after-change-functions`

(en/decode_coding_object): Fix `after-change-functions`

For `en/decode-coding-string/region`, `after-change-functions`
were either not run at all, or run only after deleting the text
but not after inserting it.

* src/coding.c (decode_coding_object, encode_coding_object): Run the
after-change-functions after inserting the result.

* test/src/editfns-tests.el (sanity-check-change-functions-with-op):
New macro.
(sanity-check-change-functions-errors): New function.
(editfns-tests--before/after-change-functions): Use them to add
cases for `en/decode-coding-string/region`.