Return values for locals, so "locals" can show them.

Also implement a couple of other JDWP commands so jdb can show the length
of arrays and the value of strings.

Change-Id: Ib2a4dc2ee784ee10bdb924e91b0150aa8a96845d
diff --git a/src/debugger.h b/src/debugger.h
index 538ebba..ec20458 100644
--- a/src/debugger.h
+++ b/src/debugger.h
@@ -171,7 +171,7 @@
   static void GetStaticFieldValue(JDWP::RefTypeId refTypeId, JDWP::FieldId fieldId, JDWP::ExpandBuf* pReply);
   static void SetStaticFieldValue(JDWP::RefTypeId refTypeId, JDWP::FieldId fieldId, uint64_t rawValue, int width);
 
-  static char* StringToUtf8(JDWP::ObjectId strId);
+  static std::string StringToUtf8(JDWP::ObjectId strId);
 
   /*
    * Thread, ThreadGroup, Frame