restore old grapheme_break API

[stevengj]

@Keno, upon reflection, I think it was a mistake to break backwards compatibility for utf8proc_grapheme_break in #70. This will make life harder for distros like Debian who need to support older versions of software (see #72).

Can we restore the old utf8proc_grapheme_break API, with the caveat that it will return incorrect results for some Unicode-9 cases, and add a new utf8proc_grapheme_break_stateful function (or similar) with the new API?

[Keno]

Yes, I'm fine with that.

[stevengj]

Can you make a PR?

[stevengj]

Closed by #76