stabilize core::option

Per API meeting

  https://github.com/rust-lang/meeting-minutes/blob/master/Meeting-API-review-2014-08-13.md

Most of the module is marked as stable or unstable; most of the unstable
items are awaiting resolution of conventions issues.

However, a few methods have been deprecated, either due to lack of use
or redundancy:

* `take_unwrap`, `get_ref` and `get_mut_ref` (redundant, and we prefer
  for this functionality to go through an explicit .unwrap)
* `filtered` and `while`
* `mutate` and `mutate_or_set`
* `collect`: this functionality is being moved to a new `FromIterator`
  impl.

Due to deprecations, this is a:

[breaking-change]
Fallout from stabilizing core::option
stabilize core::result

Per API meeting

  https://github.com/rust-lang/meeting-minutes/blob/master/Meeting-API-review-2014-08-13.md

Most of the module is marked as stable or unstable; most of the unstable
items are awaiting resolution of conventions issues.

* `collect`: this functionality is being moved to a new `FromIterator`
  impl.
* `fold_` is deprecated due to lack of use
* Several methods found in `core::option` are added here, including
  `iter`, `as_slice`, and variants.

Due to deprecations, this is a:

[breaking-change]
auto merge of #16664 : aturon/rust/stabilize-option-result, r=alexcrichton

Per API meeting

  https://github.com/rust-lang/meeting-minutes/blob/master/Meeting-API-review-2014-08-13.md

# Changes to `core::option`

Most of the module is marked as stable or unstable; most of the unstable items are awaiting resolution of conventions issues.

However, a few methods have been deprecated, either due to lack of use or redundancy:

* `take_unwrap`, `get_ref` and `get_mut_ref` (redundant, and we prefer for this functionality to go through an explicit .unwrap)
* `filtered` and `while`
* `mutate` and `mutate_or_set`
* `collect`: this functionality is being moved to a new `FromIterator` impl.

# Changes to `core::result`

Most of the module is marked as stable or unstable; most of the unstable items are awaiting resolution of conventions issues.

* `collect`: this functionality is being moved to a new `FromIterator` impl.
* `fold_` is deprecated due to lack of use
* Several methods found in `core::option` are added here, including `iter`, `as_slice`, and variants.

Due to deprecations, this is a:

[breaking-change]