Remove mbstring as a dependency

Half done with #38, but 2 of the 3 uses are removed.

Found some weird hack for string length that involves regex. I'll take it.

Not sure what to do about `mb_strtoupper()` though.

https://github.com/thephpleague/commonmark/search?utf8=%E2%9C%93&q=mb_strtoupper&type=Code