Reduce Module Dependency

[mossid]

Currently modules under x/ have following dependencies:

• Bank: Auth for accessing accounts
• Gov: Bank for modifying balances
• Slashing: Params for retrieving parameters
• Stake: Bank for modifying balances

To reduce the dependencies, we must import interface that satisfies the needs, not the keeper struct itself. We are doing it for stake module, and allso see #1528

So instead of importing bank.CoinKeeper from bank, we can define type CoinKeeper interface in types/(since it is needed widely) and import that.

[rigelrozanski]

As per discussions with @sunnya97 we should probably define a new layer of abstraction for the common interfaces which all modules anticipated to be working together fulfill (kind of like a types/ dir for a group of modules). You should sync with sunny on this one for sure!

[jackzampolin]

This is a good one! And should be part of any code cleanup!

[jackzampolin]

We have started to express this with the expected keepers concept. Going to go ahead and close this now.