Issue #24619: Simplify async/await tokenization.

This commit simplifies async/await tokenization in tokenizer.c,
tokenize.py & lib2to3/tokenize.py.  Previous solution was to keep
a stack of async-def & def blocks, whereas the new approach is just
to remember position of the outermost async-def block.

This change won't bring any parsing performance improvements, but
it makes the code much easier to read and validate.

Lib/test/test_coroutines.py

@@ -330,6 +330,7 @@ class AsyncBadSyntaxTest(unittest.TestCase):
         async def f():
             async def g(): pass
             await z
+        await = 1
         self.assertTrue(inspect.iscoroutinefunction(f))