Final alpha stabilization of core::iter

This commit wraps up the adjustments to the iterator for recent language
changes.

* Moves `rposition` from `ExactSizeIterator` to `IteratorExt` using a
  `where` clause, thereby removing the `ExactSizeIterator:
  DoubleEndedIterator` constraint.

* Merges `MutableDoubleEndedIterator` into `IteratorExt`, renaming
  `reverse_` to `reverse_in_place`.

* Merges `IteratorOrdExt`, `IteratorCloneExt` and `CloneIteratorExt`
  into `IteratorExt` using `where` clauses.

Marks as `#[stable]`:

* the `iter` module itself
* `FromIterator`, `Extend`
* `Iterator`, `IteratorExt`
* `map`
* `filter`
* `filter_map`
* `skip_while`
* `take_while`
* `scan`
* `flat_map`
* `inspect`
* `collect`
* `fold`
* `all`
* `any`
* `find`
* `rposition`
* `max`, `min`
* Various adapter types related to the above methods

Because of the trait merging, this is a:

[breaking-change]