Adds support for PEP-3132 'Extended Iterable Unpacking', fix several other bugs, and add tests for these.
Support assignments of the form a, *b, c = 1, 2, 3, 4, 5, after which b is [2, 3, 4].
Fix assignments of the form a = b = 7, a = b = 3, 4.
Fix assignment from a single-element tuple: foo = (3,) makes foo a tuple, not int.
Fix unpacking into a single-element tuple: foo, =  makes foo an int, not a list.
Fix unpacking of nested tuples: foo, (bar, baz) = 2, ('a', 5.5).
Declaration aliasing works for simple "a = b" assignment, e.g.
def aaa(a: int): return "a" bbb = aaa
It _doesn't_ work for anything more advanced, e.g.
def aaa(a: int): return "a" bbb, ccc = aaa, 4
although the function type is preserved.
This is a regression, aliasing works for non-nested tuple assignment without this patch. Sven told me not to worry about that yet...