Remove the following fields from tok_state structure which are now
used unused:

* altwarning: "Issue warning if alternate tabs don't match"
* alterror: "Issue error if alternate tabs don't match"
* alttabsize: "Alternate tab spacing"

Replace alttabsize variable with ALTTABSIZE define.

Per PEP 492, 'async' and 'await' should become proper keywords in 3.7.

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.

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()