Thrift: Generate structural fingerprints for thrift structs.

Summary:
We are going to write a dense protocol soon that eliminates some metadata.
To prevent version conflicts, we want each structure to have a
structural fingerprint that will change whenever the struct changes
in a way that will affect the dense protocol.
This change computes those fingerprints and puts them in
the generated C++ code.

Reviewed By: aditya, mcslee

Test Plan:
Recompiled thrift.
Thrifted DebugProtoTest with old and new compilers.
Compared output.
Also ran thrift with those "cout"s uncommented,
examined the fingerprint material,
and verified the hashes.

Revert Plan: ok


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665227 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/compiler/cpp/src/parse/t_set.h b/compiler/cpp/src/parse/t_set.h
index 6e505ad..68a0029 100644
--- a/compiler/cpp/src/parse/t_set.h
+++ b/compiler/cpp/src/parse/t_set.h
@@ -27,6 +27,10 @@
     return true;
   }
 
+  virtual std::string get_fingerprint_material() const {
+    return "set<" + elem_type_->get_fingerprint_material() + ">";
+  }
+
  private:
   t_type* elem_type_;
 };