Add returning to support multiple results for function invocation.

This follows the suggestion in #10

doc/documentation.md

@@ -59,6 +59,19 @@ If you would like to call the value instead of returning it, use `mockfn.macros/
     (is (thrown? ExceptionInfo (one-fn)))))
 ```
 
+#### `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.
+
+```clj
+(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)))))
+```
+
 #### `fall-through`
 
 When mocking a function, it is sometimes useful to allow calls with specific

src/mockfn/macros.cljc

@@ -32,3 +32,13 @@
          (doseq [mock# ~(into [] (keys specs#))]
            (mock/verify mock#))
          res#))))
+
+(defn returning
+  "Returns one value after the other. Used with `calling`"
+  [& values]
+  (let [stack (atom (into [] (reverse values)))
+        next (fn [_] (let [ret (peek @stack)]
+                       (when-not (empty? @stack)
+                         (swap! stack pop))
+                       ret))]
+    (calling next)))

test/mockfn/examples/basic_usage.cljc

@@ -25,6 +25,12 @@
       (is (thrown? ExceptionInfo (one-fn :argument-1)))
       (is (= :result-2 (one-fn :argument-2)))))
 
+  (testing "providing - cause function returns different results"
+    (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)))))
+
   (testing "providing with more than one function"
     (mfn/providing [(one-fn :argument) :result-1
                     (other-fn :argument) :result-2]