This is the way.
On a more serious note, there's no reason to keep adding ref-counted
classes to LibWeb now that the majority of classes is GC'd - it only
adds the risk of discovering some cycle down the line, and forces us to
use handles as we can't visit().
(And BrowsingContextGroup had to come along for the ride as well.)
This solves a number of nasty reference cycles between browsing
contexts, history items, and their documents.
With the addition of the 'fetch params' struct, the single ownership
model we had so far falls apart completely.
Additionally, this works nicely for FilteredResponse's internal response
instead of risking a dangling reference.
Replacing the public constructor with a create() function also found a
few instances of a Request being stack-allocated!
