std: add pattern matching to OsStr

Implement Haystack for &OsStr and Pattern<&OsStr> for &str, char and
Predicate.  Furthermore, add prefix/suffix matching/stripping and
splitting methods to OsStr type to make use of those patterns.

Using OsStr as a pattern is *not* implemented.  Neither is indexing
into OsStr.  All matching and indexing has to be done via provided
functions.

Author: mina86