Spec traversals can now specify a topological ordering. A topologically-
ordered traversal with input specs X1, X2... will
* include all of X1, X2... and their children
* be ordered such that a given node is guaranteed to appear before any
of its children in the traversal
Other notes:
* Input specs can be children of other input specs (this is useful if
a user specifies a set of specs to uninstall: some of those specs
might be children of others)
* `direction="parents"` will produce a reversed topological order
(children always come before parents).
* `cover="edges"` will generate a list of edges L such that (a) input
edges will always appear before output edges and (b) if you create
a list with the destination of each edge in L the result is
topologically ordered
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