SI-6642 Code cleanup from review of iteratorFrom

This commit is code cleanup from the review on
https://github.com/scala/scala/pull/2119

* Changes several instances of
    def f[A, ]… to
    def f[A]…
* Changes several instances of
    def f[A](…)(implicit ordering : Ordering[A])… to
    def f[A: Ordering](…)…
* Changes one instance of
    x == null to
    x eq null
* Changes two instances of
    id + bottomid to
    bottomid + id