Remove `NonZero` from the minting smart constructor

[carlhammann]

This PR

• removes the confusing (and unlawful) mintsListIso. We can just use txSkelMintsToList and txSkelMintsFromList, without the mental overhead associated with a non-Iso.
• changes the type of txSkelMintsFromList and similar functions by replacing NonZero Integer with Integer. Since we have to check whether things add up to zero anyway once we start using addToTxSkelMints (which txSkelMintsFromList and the Semigroup instance of TxSkelMints do), the NonZero type doesn't really enforce a valuable invariant, and only adds noise to the interface.

[carlhammann]

Funnily, NonZero never enforced anything — it's more of documentation for the reader (and a modifier for quickcheck, originally).

Yes, I was using the word rather loosely...