Remove HeapGCValue

It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.

tests/compiletest/plugin/compile-fail/ban-domrefcell.rs

@@ -13,7 +13,7 @@ use script::dom::node::Node;
 
 struct Foo {
     bar: DOMRefCell<JS<Node>>
-    //~^ ERROR Banned type DOMRefCell<JS<T>> detected. Use MutHeap<JS<T>> instead,
+    //~^ ERROR Banned type DOMRefCell<JS<T>> detected. Use MutJS<JS<T>> instead,
 }
 
 fn main() {}

tests/compiletest/plugin/compile-fail/ban.rs

@@ -12,7 +12,7 @@ use std::cell::Cell;
 
 struct Foo {
     bar: Cell<JSVal>
-    //~^ ERROR Banned type Cell<JSVal> detected. Use MutHeap<JSVal> instead,
+    //~^ ERROR Banned type Cell<JSVal> detected. Use MutJS<JSVal> instead,
 }
 
 fn main() {}