Optimize global object access

[toshok]

IMO ejs shouldn't allow a writable global object, and all normal JS objects should be non-writable as well, at least by default. If we disable all writes (i.e. globals are frozen) then we can inline accesses for a ton of things at compile time.

I.e.

let a = Object.create (null);

This gets transformed to contain %getGlobal(Object) (which is itself a property lookup, just with implicit obj parameter) + a normal property lookup.

If everything is frozen, then we could directly invoke the function _ejs_Object_create (not even using _ejs_invoke_closure, since we can annotate the function such that the compiler knows it doesn't close over anything.)