Add ContractAddr generic helper

[ethanfrey]

Based on discussion from #150 (comment)

Add pub type ContractAddr(pub HumanAddr) to cw0, with some common functions to create messages and make queries, as well as

Rename Cw4Contract to Cw4ContractAddr, and extend ContractAddr functionality. See if the refactoring simplifies the implementation. If not, we can just do the rename without adding some ContractAddr type (I am not sure if it would make anything easier).

If this simplifies things, do the same for the other helpers (cw3, cw20, cw721?).

[ethanfrey]

Note, looking more at the helpers, I am unsure if this extra type will help much. It may allow us to de-dup encode_msg, encode_smart_query and encode_raw_query helper methods, but the creation may be more work. eg we have to implement From<HumanAddr> to avoid ugly init: let addr = Cw4ContractAddr(ContractAddr(msg.contract)); and rather use: let addr = Cw4ContractAddr::from(msg.contract)

[ethanfrey]

I would close this unless you still think this is valuable.

[maurolacy]

I agree this is OK as such.