Use decode_string_utf_8 in emacs-module.c.

Use decode_string_utf_8 in emacs-module.c.

Now that decode_string_utf_8 is available, we can use it to signal
errors on invalid input.

* src/coding.c (syms_of_coding): Move Qutf_8_string_p from json.c
since it’s now used outside json.c.

* src/emacs-module.c (module_decode_utf_8): New helper function.
(module_make_function, module_copy_string_contents): Use it.