Add returning to support multiple results for function invocation.

[phmeier-nubank]

This follows the suggestion in #10. Excerpt from the docs:

returning

If you want to return different values for each invocation, use mockfn.macros/returning.
Note that if the function will return nil afer all provided results are consumed.

(testing "providing - cause function non referential"
    (mfn/providing [(one-fn :argument-1) (mfn/returning :result-1 :result-2 )]
                   (is (= :result-1 (one-fn :argument-1)))
                   (is (= :result-2 (one-fn :argument-1)))
                   (is (nil? (one-fn :argument-1)))))