User should not be able to both slash and stake an argument

[truted]

• reject Staking msg if argument is already slashed
• reject Slashing msg if argument is already staked

[shanev]

We are handling this on the client side, but will be good to get this in at some point.

[shanev]

To make this work, we have to generalize the slashing module (add expected keepers, interfaces, exported types and aliases). Then staking can depend on slashing, and slashing can depend on staking without cyclic dependencies.