txout are single tx only

Thorchain shouldn't know about multisend, as it is chain dependent, and we want to keep thorchain as chain agnostic as possible.

Instead, each tx can only have one coin. When the signer gets a list of all tx, the signer can decide (relative to the chain) what it can batch up into a single tx and what it cannot.

Eventually, we will remove the concept of predicting gas fees, and let the result of the tx inform thorchain what the gas was. This is more flexible as different chains calculate fees differently. I will be doing this work in a follow on PR.