Refine bindings #2

[maurolacy]

Related to confio/osmosis#4. Some changes required for implementing EstimatePrice:

• The multihop swap methods (https://github.com/confio/osmosis/blob/72dee1fd3b5760ca740f1125ee81fd9a06757178/x/gamm/keeper/multihop.go#L12 and https://github.com/confio/osmosis/blob/72dee1fd3b5760ca740f1125ee81fd9a06757178/x/gamm/keeper/multihop.go#L38) require a OutMin / InMax parameter. Compatible with SwapAmountWithLimit).
• The denom field in the Step struct is being used as both, denom_out and denom_in, depending on the swap type. Renaming to denom for generality.
• These methods also require a sender (contract) address. Not sure why. Not sure also if that can be obtained on the Go side, from the Context or so.

[ethanfrey]

I like the code refactor.

I do not agree with the type changes. I would revert both.
You can merge with just the logic refactor or add the sender parameter as we seem to need that more than anything.