[ADP-3272] Use `Data.Set.NonEmpty` for `UnresolvedInputs` error.

[jonathanknowles]

This PR adjusts the ErrBalanceTxUnresolvedInputs error to use a non-empty set instead of a non-empty list:

- | ErrBalanceTxUnresolvedInputs (NonEmpty TxIn)
+ | ErrBalanceTxUnresolvedInputs (NESet    TxIn)

Justification

• Transaction inputs are unique.
• The ability to represent duplicates is neither necessary nor helpful.
• The order of inputs is unimportant.

Issue

ADP-3272