Fix issues with ast slices.
Fix issues with ast slices.

Empty slices (x[::]) were not properly handled, and there was an extra node is the produces tree for slices.

IronLanguages/ironpython2,slozier/ironpython2,IronLanguages/ironpython2,IronLanguages/ironpython2,slozier/ironpython2,slozier/ironpython2,IronLanguages/ironpython2,slozier/ironpython2,IronLanguages/ironpython2,IronLanguages/ironpython2,slozier/ironpython2,slozier/ironpython2,IronLanguages/ironpython2,slozier/ironpython2