Hedge v2 balancing WIP

[carlhammann]

My last commit tries to reimplement calcBalanceTx in terms of SpendableOuts and TxSkels. This function is now independent of Cooked.Tx.Balance. Work that remains (and that I can see):

• adapt applyBalanceTx to the new types
• try to use these new balancing primitives (before generateCardanoBuildTx!)

[carlhammann]

I propose we merge this now. The work is far from finished, but the goal that we set ourselves here is reached: We're successfully balancing transactions for the auction contract, all of whose single-trace test cases now exhibit the same behaviour they show on the main branch.