CoinMapper is now called CoinKeeper to differentiate it from
AccountMapper. A Mapper retrieves things from a store. A Keeper exposes
functionality of coins and maintain invariants.
bank.NewHandler takes a CoinKeeper instead of the entire AccountMapper.
Refactor
* No more decorators, but rather types.AntiHandler
* No more handlers, but rather types.MsgHandler
* Ability to pass "stores" in NewXYZHandler()
* Coins live in types, and Accounts have coins
* Coinstore -> bank