validation for star expressions should not be done in the parser

[slozier]

For example, x = ast.parse("a = 1,*b") should succeed and only fail when compiling compile(x, None, 'exec').

See #662 (comment)

[slozier]

#1158 resolves the parsing, but the expression fails to compile with a TypeError:

import ast
x = ast.parse("a = 1,*b")
compile(x, "", 'exec')

TypeError: Unexpected expr type: IronPython.Modules._ast+Starred