Issue #24400: Introduce a distinct type for 'async def' coroutines.

Summary of changes:

1. Coroutines now have a distinct, separate from generators
   type at the C level: PyGen_Type, and a new typedef PyCoroObject.
   PyCoroObject shares the initial segment of struct layout with
   PyGenObject, making it possible to reuse existing generators
   machinery.  The new type is exposed as 'types.CoroutineType'.

   As a consequence of having a new type, CO_GENERATOR flag is
   no longer applied to coroutines.

2. Having a separate type for coroutines made it possible to add
   an __await__ method to the type.  Although it is not used by the
   interpreter (see details on that below), it makes coroutines
   naturally (without using __instancecheck__) conform to
   collections.abc.Coroutine and collections.abc.Awaitable ABCs.

   [The __instancecheck__ is still used for generator-based
   coroutines, as we don't want to add __await__ for generators.]

3. Add new opcode: GET_YIELD_FROM_ITER.  The opcode is needed to
   allow passing native coroutines to the YIELD_FROM opcode.

   Before this change, 'yield from o' expression was compiled to:

      (o)
      GET_ITER
      LOAD_CONST
      YIELD_FROM

   Now, we use GET_YIELD_FROM_ITER instead of GET_ITER.

   The reason for adding a new opcode is that GET_ITER is used
   in some contexts (such as 'for .. in' loops) where passing
   a coroutine object is invalid.

4. Add two new introspection functions to the inspec module:
   getcoroutinestate(c) and getcoroutinelocals(c).

5. inspect.iscoroutine(o) is updated to test if 'o' is a native
   coroutine object.  Before this commit it used abc.Coroutine,
   and it was requested to update inspect.isgenerator(o) to use
   abc.Generator; it was decided, however, that inspect functions
   should really be tailored for checking for native types.

6. sys.set_coroutine_wrapper(w) API is updated to work with only
   native coroutines.  Since types.coroutine decorator supports
   any type of callables now, it would be confusing that it does
   not work for all types of coroutines.

7. Exceptions logic in generators C implementation was updated
   to raise clearer messages for coroutines:

   Before: TypeError("generator raised StopIteration")
   After: TypeError("coroutine raised StopIteration")

Doc/reference/compound_stmts.rst

@@ -666,6 +666,15 @@ can be used to create instance variables with different implementation details.
 Coroutines
 ==========
 
+.. index::
+   statement: async def
+   statement: async for
+   statement: async with
+   keyword: async
+   keyword: await
+
+.. versionadded:: 3.5
+
 .. _`async def`:
 
 Coroutine function definition
@@ -683,7 +692,11 @@ even if they do not contain ``await`` or ``async`` keywords.
 
 It is a :exc:`SyntaxError` to use :keyword:`yield` expressions in coroutines.
 
-.. versionadded:: 3.5
+An example of a coroutine function::
+
+    async def func(param1, param2):
+        do_stuff()
+        await some_coroutine()
 
 
 .. _`async for`:
@@ -725,7 +738,8 @@ Is semantically equivalent to::
 
 See also :meth:`__aiter__` and :meth:`__anext__` for details.
 
-.. versionadded:: 3.5
+It is a :exc:`SyntaxError` to use ``async for`` statement outside of an
+:keyword:`async def` function.
 
 
 .. _`async with`:
@@ -762,7 +776,8 @@ Is semantically equivalent to::
 
 See also :meth:`__aenter__` and :meth:`__aexit__` for details.
 
-.. versionadded:: 3.5
+It is a :exc:`SyntaxError` to use ``async with`` statement outside of an
+:keyword:`async def` function.
 
 .. seealso::