`gfiltermap` can result in inconsistent graph.

Consider this code:

isLeaf :: Context a b -> Bool
isLeaf (_, _, _, [ ]) = True
isLeaf (_, _, _, _ ) = False

tearLeaves = let p x = if isLeaf x then Nothing else Just x in gfiltermap p

Example:

λ dag3
mkGraph [(1,'a'),(2,'b'),(3,'c')] [(1,3,())]
λ tearLeaves dag3
mkGraph [(1,'a')] [(1,3,())]

As you see, we obtain a hanging edge.

I propose `gfiltermap` is improved so that validity of graphs is preserved.