Simplify Fee Handling to use 0 or 1 Coin, not N

[ethanfrey]

There are two different approaches proposed here and in the following issue. Maybe we allow both (as a Param config), but I think best is to choose one. This one is just a minor simplification of the logic:

I found it odd that we change Coin[]. I mean, we could charge 10 OSMO, 3 ION and 1 ibc-transfered ATOM via channel-7 ??? This field allows for N coins. I don't imagine any realistic use-case with N different than 0 or 1. I would rather represent that as *Coin (nil for 0) or simply Coin, where coin.amount.IsZero() represents "no fee".