This speeds up ScriptRequest, particularly in JSNative calls, i.e. particularly in e.g. calling QueryInterface from JS;
In general, storing the global and the native scope is not necessary. We can just recompute them on the fly where needed (which is not generally 'hot' code).
This makes this faster by default, but it also makes it much easier to introduce a fast-path for JSNative.
Indeed, when we are called from JS, we can be sure we are already in the caller-realm, so no need to change anything.
In some profiling, this cuts about 10% of runtime to the wrapper call in QueryInterface.
TODO: we could speed things up slightly more by doing the formerRealm stuff at compile-time, and essentially just pass a pointer around, which we could maybe to by value.