toString() implementation for iterators

[scristalli]

Iterators (such as Immutable, see #1102) do not have an implementation for toString. It should be decided whether their representation should be the whole collection (as for Iterable), or just a textual representation that does not require traversing the iterator itself.

[0crat]

@llorllale/z please, pay attention to this issue

[llorllale]

@scristalli good question - if Iterator.toString() prints out all elements, it would consume the iterator itself.