std: Fully stabilize Option<T>

This commit takes a second pass through the `std::option` module to fully
stabilize any lingering methods inside of it.

These items were made stable as-is

* Some
* None
* as_mut
* expect
* unwrap
* unwrap_or
* unwrap_or_else
* map
* map_or
* map_or_else
* and_then
* or_else
* unwrap_or_default
* Default implementation
* FromIterator implementation
* Copy implementation

These items were made stable with modifications

* iter - now returns a struct called Iter
* iter_mut - now returns a struct called IterMut
* into_iter - now returns a struct called IntoIter, Clone is never implemented

This is a breaking change due to the modifications to the names of the iterator
types returned. Code referencing the old names should updated to referencing the
newer names instead. This is also a breaking change due to the fact that
`IntoIter` no longer implements the `Clone` trait.

These items were explicitly not stabilized

* as_slice - waiting on indexing conventions
* as_mut_slice - waiting on conventions with as_slice as well
* cloned - the API was still just recently added
* ok_or - API remains experimental
* ok_or_else - API remains experimental

[breaking-change]
Merge pull request #19849 from alexcrichton/second-pass-option

std: Fully stabilize Option<T>

Reviewed-by: aturon