add is_alphabetic to the UnicodeStrPrelude trait

[pwoolcoc]

No description provided.

[alexcrichton]

@aturon and I have actually talked quite a bit this week about the stabilization of many modules, including str! We currently plan to remove these convenience methods as we provide an inconsistent listing of them with respect to those provided on char, and they're easy enough to call through iterators. For now I'm going to close this (stabilization PR to appear shortly hopefully!), but thanks for the PR!