Implement set trap for proxies, and revamp class hierarchy in preparation

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index f0e7bbcfe02885761e96ddf510e2ede52bfb7a53..2b3b0acdec5309d421ab6c9f227cbc4d0284e22f 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1292,6 +1292,7 @@ void Genesis::InstallNativeFunctions() {
 void Genesis::InstallExperimentalNativeFunctions() {
   if (FLAG_harmony_proxies) {
     INSTALL_NATIVE(JSFunction, "DerivedGetTrap", derived_get_trap);
+    INSTALL_NATIVE(JSFunction, "DerivedSetTrap", derived_set_trap);
   }
 }