Unnecessary constraints in the `Graph` class

[fiedlerp]

The constraints (Hashable v, Eq v) for methods of the Graph class are totally unnecessary. These constraints are implementation-specific for UGraph and DGraph, so the constraints should be moved to instances of the Graph.

instance (Hashable v, Eq v) => Graph (Graph v e) where ...

Of course, the Eq v makes sense at some of the methods (e.g. removeVertex) but definitely not all.

[alx741]

Totally agree, will try and refactor that as soon as I have the time. If you'd like to try a PR for this though, it's welcome :) if not I'll try to make time for this soon, it's been stalled sadly