Use Coin Enum instead of Option<Addr>

[JakeHartnell]

Having a token enum, would be cleaner than having Optional addresses representing Native Tokens / Cw20s.

https://github.com/Wasmswap/Junoswap/blob/18092a733281622e8dec8f0a23c8d343dae1b8a5/src/msg.rs#L10

[JakeHartnell]

Use this for Token state as well.