re2: import various changes from Google's copy

Change-Id: I63bd284d742da393713dfdd0164d2ff38fd44920
Reviewed-on: https://code-review.googlesource.com/1592
Reviewed-by: Russ Cox <rsc@swtch.com>

re2/stringpiece.h

diff --git a/re2/stringpiece.h b/re2/stringpiece.h
index ab9297c..37c73d1 100644
--- a/re2/stringpiece.h
+++ b/re2/stringpiece.h
@@ -138,6 +138,8 @@
 
   int copy(char* buf, size_type n, size_type pos = 0) const;
 
+  bool contains(StringPiece s) const;
+
   int find(const StringPiece& s, size_type pos = 0) const;
   int find(char c, size_type pos = 0) const;
   int rfind(const StringPiece& s, size_type pos = npos) const;