Tests of the ``closure`` function; just need to verify that
it correctly handles positional and keyword arguments, alone
or in combination.

    >>> from plib.stdlib import closure
    >>> f = closure(sum, range(10))
    >>> f()
    45
    >>> f = closure(dict, a=1, b=2)
    >>> f() == {'a': 1, 'b': 2}
    True
    >>> f = closure(dict, {'a':1, 'b':2}, c=3, d=4)
    >>> f() == {'a': 1, 'b': 2, 'c': 3, 'd': 4}
    True
