Migrate Wallet::build_fee_bump to a TxBuilder method.

[LLFourn]

This is unfinished business from #258.

Right now you can only bump the fee of one tx at a time by creating a fee bump TxBuilder by calling Wallet::build_fee_bump . Why not allow bumping the fee of multiple transactions?

I think this can be done by just having a bump_fee method on TxBuilder which takes a transaction and adds all the inputs and outputs to the internal state and recording the minimum fee for the fee bump to work.