Issue #24619: New approach for tokenizing async/await.

This commit fixes how one-line async-defs and defs are tracked
by tokenizer.  It allows to correctly parse invalid code such
as:

>>> async def f():
...     def g(): pass
...     async = 10

and valid code such as:

>>> async def f():
...     async def g(): pass
...     await z

As a consequence, is is now possible to have one-line
'async def foo(): await ..' functions:

>>> async def foo(): return await bar()

Lib/test/badsyntax_async2.py

@@ -1,3 +1,2 @@
-async def foo():
-    def foo(a:await something()):
-        pass
+async def foo(a:await something()):
+    pass