feat(pipe): isolate pipe operator overload code

tests/pipe_test.py

@@ -0,0 +1,47 @@
+# -*- coding: utf-8 -*-
+import asyncio
+import pytest
+import paco
+from paco.pipe import overload
+
+
+def test_pipe_operator_overload():
+    @asyncio.coroutine
+    def filterer(x):
+        return x < 8
+
+    @asyncio.coroutine
+    def mapper(x):
+        return x * 2
+
+    @asyncio.coroutine
+    def drop(x):
+        return x < 10
+
+    @asyncio.coroutine
+    def reducer(acc, x):
+        return acc + x
+
+    @asyncio.coroutine
+    def task(numbers):
+        return (yield from (numbers
+                            | paco.filter(filterer)
+                            | paco.map(mapper)
+                            | paco.dropwhile(drop)
+                            | paco.reduce(reducer, initializer=0)))
+
+    result = paco.run(task((1, 2, 3, 4, 5, 6, 7, 8, 9, 10)))
+    assert result == 36
+
+
+def test_overload_error():
+    with pytest.raises(TypeError, message='fn must be a callable object'):
+        overload(None)
+
+    with pytest.raises(ValueError,
+                       messsage='invalid function signature or arity'):
+        overload(lambda x: True)
+
+    with pytest.raises(ValueError,
+                       messsage='invalid function signature or arity'):
+        overload(lambda x, y: True)