Make "query" entry point optional

[webmaster128]

After #698 it becomes very easy to make the query entry point optional (just like migrate). This is probably used by most contract and best practice to have, but making it optional shows the power of the more dynamic import system and avoids placeholder implementations like this:

pub fn query(_deps: Deps, _env: Env, _msg: QueryMsg) -> StdResult<QueryResponse> {
    Err(StdError::generic_err(
        "You can only use this contract for migrations",
    ))
}