We now precisely track flow from positional arguments to splat
parameters, provided that splat arguments are not used and there are no
positional parameters after the splat parameter. For example, in this
case:

    def f(x, y, *z); end

    f(a, b, c, d)

we get flow from `c` to `z[0]` and `d` to `z[1]`.

We get false flow if there are positional parameters after the splat
parameter. For example in this case:

    def g(x, y, *z, w); end

    g(a, b, c, d)

we get flow from `d` to `z[0]` instead of `w`.

We also track flow in this case

    def f(a, *b)
      sink b[0]
    end

    f(1, *[taint, 2])

Make this class into a proper subclass of `ParameterNodeImpl`, to
prevent some consistency test failures.