Make the `TxSkel` transaction inputs a `Map`

[carlhammann]

It's in the title. The motivation is to ensure by construction that the TxSkel spends every SpendableOut at most once.

I intend to base my re-integration of the attack language on (something like) this.

[carlhammann]

The module that defines TxSkel needs a cleanup pass. Here are a few things I can think of (maybe one small PR each?):

• At the moment, all of the RawModTx stuff does nothing. We've got to either remove it or re-implement it in some suitable way.
• Our description of collateral inputs is redundant; there's a field in the TxOpts and one separate one on the TxSkel.
• In general, I'm not happy that fees and collaterals are so visible on the TxSkel. They should be hidden from the end user. I see at least the following options:
  • Hide them in an intermediate type for use only in balancing.
  • Use the TxOpts or some other clearly "only touch this if you know what you're doing" area.
• Do we still need all of that business around TxSkel forming a Semigroup? (I think it might just be of use in writing tests for generateTxBodyContent'.)
• The whole module is lacking documentation.
• It's at the wrong place, since Cooked.Tx.Constraints.Type is not anymore what the module is about.