Generate visualization of counterexample

[gridaphobe]

In addition to

λ> target (insert0 :: E -> T -> T) 'insert0 "examples/AVL.hs"
Found counter-example: 'e', Node {key = 'a', l = Leaf, r = Node {key =
'd', l = Leaf, r = Leaf, ah = 1}, ah = 2}

it would be nice to get e.g. a graphviz diagram of the tree. Shouldn't be too difficult, just convert the value to an Expr and generate the graphviz from that.