Remove the Show instances for monadic graphs

[treeowl]

They are not even remotely legitimate and there should never be any reason to use them. IMO, they should be removed promptly in the next major version.

[treeowl]

We should instead offer a safe version that works in MonadIO.

[ivan-m]

This will have to be a major version bump in case anyone is actually using/relying upon them.

But I agree; the unsafePerformIO squicks me out.