rustdoc: Remove outdated syntax

Fixing some of rustdoc's rendering to use newer syntaxes rather than older
syntaxes.
auto merge of #14957 : alexcrichton/rust/rustdoc-fixups, r=huonw

Fixing some of rustdoc's rendering to use newer syntaxes rather than older
syntaxes.
librustc: Check regions for overloaded calls.
auto merge of #14976 : luqmana/rust/focwtc, r=pcwalton

Fixes #14959.
Revamp TaskBuilder API

This patch consolidates and cleans up the task spawning APIs:

* Removes the problematic `future_result` method from `std::task::TaskBuilder`,
  and adds a `try_future` that both spawns the task and returns a future
  representing its eventual result (or failure).

* Removes the public `opts` field from `TaskBuilder`, instead adding appropriate
  builder methods to configure the task.

* Adds extension traits to libgreen and libnative that add methods to
  `TaskBuilder` for spawning the task as a green or native thread.

Previously, there was no way to benefit from the `TaskBuilder` functionality and
also set the scheduler to spawn within.

With this change, all task spawning scenarios are supported through the
`TaskBuilder` interface.

Closes #3725.

[breaking-change]
Fallout from TaskBuilder changes

This commit brings code downstream of libstd up to date with the new
TaskBuilder API.