[WIP] Add Coroutine ABC to PEP 484 (#125)

* Simpler example for Coroutine

pep-0484.txt

@@ -1254,7 +1254,20 @@ type of ``await`` expression, not to the coroutine type::
   async def foo() -> None:
       bar = await spam(42)  # type: str
 
-The ``typing.py`` module also provides generic ABCs ``Awaitable``,
+The ``typing.py`` module provides a generic version of ABC
+``collections.abc.Coroutine`` to specify awaitables that also support
+``send()`` and ``throw()`` methods. The variance and order of type variables
+correspond to those of ``Generator``, namely ``Coroutine[T_co, T_contra, V_co]``,
+for example::
+
+  from typing import List, Coroutine
+  c = None # type: Coroutine[List[str], str, int]
+  ...
+  x = c.send('hi') # type: List[str]
+  async def bar(): -> None:
+      x = await c # type: int
+
+The module also provides generic ABCs ``Awaitable``,
 ``AsyncIterable``, and ``AsyncIterator`` for situations where more precise
 types cannot be specified::
 
@@ -1768,10 +1781,14 @@ Generic variants of container ABCs (and a few non-containers):
 
 * Callable (see above, listed here for completeness)
 
+* Collection
+
 * Container
 
 * ContextManager
 
+* Coroutine
+
 * Generator, used as ``Generator[yield_type, send_type,
   return_type]``.  This represents the return value of generator
   functions.  It is a subtype of ``Iterable`` and it has additional