Distinguish between weight 0 and not member in cw3 queries

[ethanfrey]

Cw3QueryMsg::Voter{addr: HumanAddr} should return something like cw4::MemberResponse, eg:

pub struct VoterInfo {
  pub weight: Option<u64>,
}

Do not change the VoterListResponse return info.

[maurolacy]

Do not change the VoterListResponse return info.

Hmm, I see now that I missed that restriction. In any case, re-open and I'll revisit this: basically, by using two different (sub)structs for VoterResponse and VoterListResponse.

[ethanfrey]

I missed that too just looking at the diff.

The member list always has a value, to avoid needless unwraps

[maurolacy]

Take a look at #159, and sorry for the confusion.