Bug 789897 - Implement isExtensible and preventExtensions traps for proxies, including scriptable direct proxies, and in the various wrapper classes.  Also move non-static JSObject::preventExtensions to static ObjectImpl::preventExtensions, for handle correctness.  Patch is a semi-tag-team effort with jwalden.  r=jwalden, r=ejpbruel, r=bholley, r=two-turtle-doves-and-a-partridge-in-a-pear-tree

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extra : rebase_source : 156d2c22499372fceb60d130e363a3685e68b9fc