Porting/bench -- utf8 to code point speed up

I think the API in 5.26 for handling UTF-8 is finally good enough that 
it's time to work on speeding up.

This free dfa converter has long been known to us, and is considered the 
one to beat by various people on the Unicode mailing list

http://bjoern.hoehrmann.de/utf-8/decoder/dfa/

• Porting/bench -- utf8 to code point speed up by Karl Williamson