bug fix: polymorphic equality in leaderlist module

[smolkaj]

Polymorphic equality raises a runtime exception in some situations, for example when instantiating this module with a Imperative.Digraph.AbstractLabeled graph:

> module G = Graph.Imperative.Digraph.AbstractLabeled(V)(E)
> module L = Graph.Leaderlist.Make(G)
> val g : G.t
> val v : G.V.t
> Leaderlist.leader_lists g v
Uncaught exception:
  
  (Invalid_argument "compare: functional value")

Raised by primitive operation at file "src/leaderlist.ml", line 48, characters 9-17
Called from file "src/leaderlist.ml", line 42, characters 14-17
...

[yakobowski]

Indeed, thanks!

[yakobowski]

Merging that one, I will prepare another PR which fixes various other faulty occurrences.

[smolkaj]

That was fast. Thanks!