Merge InitResponse/HandleResponse/MigrateResponse

[ethanfrey]

These all have identical forms now and in the forseeable future. I will also add two more such calls, to return SystemResponse and CallbackResponse which also have the same form.

Do we really need to maintain 5 differently named but identical types? It is maintenance overhead, we need to be careful to keep them in sync, and bloats the wasm code size.

Let's just make a CallResponse or ActionResponse or such and use this everywhere. We could provide type HandleResponse = ActionResponse for the transition period to make it less troublesome to migrate contracts

[webmaster128]

What about simply cosmwasm_std::Response?