Move starred expression checks out of parser

[slozier]

Moves the starred expression checks out of parser which resolves the following:

import ast
try:
    ast.parse("*a")
except SyntaxError:
    assert False

[slozier]

@BCSharp do you think this seems like a reasonable approach to post-parsing syntax checks?

[BCSharp]

I think it is a good approach. My only concern was about the ordering of the checks (it comes after namebinding syntax errors are reported), but it appears this is exactly what CPython does as well, so all is well.

A few other things that caught my eye, I'm pretty sure you know this, but just for the record:

1. Going through compile(ast.parse(...), ...) does not work, even for valid cases; I assume this is for another PR, maybe a one that finishes off #686?
2. Starting from Python 3.5, the error message "can use starred expression only as assignment target" is replaced by "can't use starred expression here", I assume this will get changed in #1153?

[slozier]

Going through compile(ast.parse(...), ...) does not work, even for valid cases; I assume this is for another PR, maybe a one that finishes off #686?

Ah, I knew there was a starred expression issue somewhere, good catch. I guess this solved half the issue. Will take a look at the second half in a separate PR (if I don't forget about it).

Starting from Python 3.5, the error message "can use starred expression only as assignment target" is replaced by "can't use starred expression here", I assume this will get changed in #1153?

Indeed, 3.5 will change a bunch of things in this walker, but it seemed like a good idea to work this out separately from the other PR.