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/lib2to3/tests/test_parser.py

@@ -67,10 +67,32 @@ class TestAsyncAwait(GrammarTest):
                              await x
                       """)
 
+        self.validate("""async def foo():
+
+            def foo(): pass
+
+            def foo(): pass
+
+            await x
+        """)
+
+        self.validate("""async def foo(): return await a""")
+
+        self.validate("""def foo():
+            def foo(): pass
+            async def foo(): await x
+        """)
+
         self.invalid_syntax("await x")
         self.invalid_syntax("""def foo():
                                    await x""")
 
+        self.invalid_syntax("""def foo():
+            def foo(): pass
+            async def foo(): pass
+            await x
+        """)
+
     def test_async_var(self):
         self.validate("""async = 1""")
         self.validate("""await = 1""")