Cosmwasm bindings for TWAP

[ValarDragon]

Background

Make a cosmwasm binding for the TWAP work.

Suggested Design

Should be a binding around the exposed functions in the x/gamm/twap/api.go file, namely GetArithmeticTwap and GetArithmeticTwapToNow.

We should keep an eye out towards how can we store our own records cosmwasm side, and then use the record based API once it exists.

Acceptance Criteria

• [ ] Cosmwasm binding for TWAP
• [ ] Example contract invoking it