Reduce namespace pollution.

This leaves us with just the mspace stuff and three libdex functions to clean
up. We deliberately expose the JII API, and I don't think there's anything we
can really do about the art_..._from_code stuff (and at least that starts with
"art_").

Change-Id: I77e58e8330cd2afeb496642302dfe3311e68091a
diff --git a/src/stringpiece.cc b/src/stringpiece.cc
index 3779550..51f3980 100644
--- a/src/stringpiece.cc
+++ b/src/stringpiece.cc
@@ -18,7 +18,7 @@
 
 namespace art {
 
-bool operator<(const art::StringPiece& x, const art::StringPiece& y) {
+bool operator<(const StringPiece& x, const StringPiece& y) {
   const int r = memcmp(x.data(), y.data(),
                        std::min(x.size(), y.size()));
   return ((r < 0) || ((r == 0) && (x.size() < y.size())));
@@ -90,7 +90,7 @@
 
 const StringPiece::size_type StringPiece::npos = size_type(-1);
 
-std::ostream& operator<<(std::ostream& o, const art::StringPiece& piece) {
+std::ostream& operator<<(std::ostream& o, const StringPiece& piece) {
   o.write(piece.data(), piece.size());
   return o;
 }