Introduce custom UTF-8 decoding pipeline.

WordPress relies on various extensions, regular expressions, and basic
string operations when working with text potentially encoded as UTF-8.

In this patch an efficient UTF-8 decoding pipeline is introduced which
can remove these dependencies, normalize all decoding behaviors, and
open up new kinds of processing opportunities.

The decoder was taken from [Björn Höhrmann]. While it may be possible
that other methods are more efficient, such as in the multi-byte
extension, this decoder provides a streamable interface useful for
more flexible kinds of processing: for example, whether or not to
replace invalid byte sequences, zero-memory-overhead code point
counting, and partially decoding strings.

[Björn Höhrmann]: http://bjoern.hoehrmann.de/utf-8/decoder/dfa/