New msgtx generation

I think it's now unnecessary to call checkMsgTxOutputs() here. I think our tests that want to check outputs on a decoratedTx should be concerned only with what's in the decoratedTx, because now our code doesn't maintain a MsgTx alongside the decoratedTx, so we don't need to check that they're in sync all the time -- all we need is good tests for toMsgTx(), which we don't have right now