Merge Data.Arity.Linear.Peano with Unsafe.INat

[tbagrel1]

The need for an alternative Nat definition arose in both Unsafe and Data.{V,Replicator}.Linear modules.

As a result, we created Peano and INat with the same definition and purpose. We should merge them.

[treeowl]

While we're at it, why does Arity produce a Nat rather than a Peano? I think we always convert the result to a Peano before we actually use it.

[aspiwack]

I think that there was a reason. Or maybe it's simply because it was a quicker refactor. But either way, I don't remember. It may be worth trying and see what feels best.