Migrate to the latest version of the UTF-8 API

[adamziel]

@dmsnell significantly improved the speed of the UTF-8 decoder, let's bring those improvements into this project.

[dmsnell]

https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/compat-utf8.php

https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/utf8.php