std: Second pass stabilization of Result<T, E>

This commit, like the second pass of `Option`, largely just stablizes the
existing functionality after renaming a few iterators.

The specific actions taken were:

* The `Ok` and `Err` variants were marked `#[stable]` as the stability
  inheritance was since removed.
* The `as_mut` method is now stable.
* The `map` method is now stable
* The `map_err` method is now stable
* The `iter`, `iter_mut`, and `into_iter` methods now returned structures named
  after the method of iteration. The methods are also now all stable.
* The `and_then` method is now stable.
* The `or_else` method is now stable.
* The `unwrap` family of functions are now all stable: `unwrap_or`,
  `unwrap_or_else`, `unwrap`, and `unwrap_err`.

There is a possible open extension to `Result::{and, and_then}` to make the
return type further generic over `FromError` (as proposed in #19078), but this
is a backwards compatible change due to the usage of default type parameters,
which makes the two functions safe to stabilize now regardless of the outcome of
that issue.
Merge pull request #19961 from alexcrichton/second-pass-result

std: Second pass stabilization of Result<T, E>

Reviewed-by: aturon