Eliminate the extra indirection of DList<~Flow>
auto merge of #1626 : kmcallister/servo/flowlist, r=pcwalton

r? @pcwalton

Some points of concern:

* Is this really memory-safe? Is there a way to access a stale `Rawlink` that doesn't exist in the normal `DList` code?
* Is it okay to coerce both `~Flow` and `&Flow` to `(uint, uint)` (or the moral equivalent) and compare them?  Will a given `Flow` always have the same (obj, vtable) regardless of whether it's obtained through `~Flow` or `&Flow`?