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

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index b0d3ae4cdc0d218bb450cc5df0f467ff5ed7d243..a89e26228150502475e29d96e017929ea61bfd4c 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -108,7 +108,8 @@ enum ContextLookupFlags {
   V(MAP_CACHE_INDEX, Object, map_cache) \
   V(CONTEXT_DATA_INDEX, Object, data) \
   V(ALLOW_CODE_GEN_FROM_STRINGS_INDEX, Object, allow_code_gen_from_strings) \
-  V(DERIVED_GET_TRAP_INDEX, JSFunction, derived_get_trap)
+  V(DERIVED_GET_TRAP_INDEX, JSFunction, derived_get_trap) \
+  V(DERIVED_SET_TRAP_INDEX, JSFunction, derived_set_trap)
 
 // JSFunctions are pairs (context, function code), sometimes also called
 // closures. A Context object is used to represent function contexts and
@@ -240,6 +241,7 @@ class Context: public FixedArray {
     CONTEXT_DATA_INDEX,
     ALLOW_CODE_GEN_FROM_STRINGS_INDEX,
     DERIVED_GET_TRAP_INDEX,
+    DERIVED_SET_TRAP_INDEX,
 
     // Properties from here are treated as weak references by the full GC.
     // Scavenge treats them as strong references.