librustc: Remove useless code to handle `let _ = ...`

This code didn't do anything, but was a vestige of the old semantics for
`let _ = ...`.

Closes #10488. (As near as I can tell anyhow.)
Small improvement for json PrettyEncoder
auto merge of #14932 : Sawyer47/rust/json-smallfix, r=huonw
auto merge of #14882 : pcwalton/rust/drop-underscore, r=nikomatsakis

This code didn't do anything, but was a vestige of the old semantics for
`let _ = ...`.

Closes #10488. (As near as I can tell anyhow.)

r? @nikomatsakis
Add examples of how to read from a channel with a timeout, refs #13862
auto merge of #14877 : Seldaek/rust/commdocs, r=alexcrichton

Finally what I promised to do in #13862 /cc @alexcrichton
std: Support consuming a Process without waiting

Forking off a child which survives the parent is often a useful task, and is
currently not possible because the Process type will invoke `wait()` in its
destructor in order to prevent leaking resources. This function adds a new safe
method, `forget`, which can be used to consume an instance of `Process` which
will then not call `wait` in the destructor.

This new method is clearly documented as a leak of resources, but it must be
forcibly opted in to.

Closes #14467
std: Improve pipe() functionality

* os::pipe() now returns IoResult<os::Pipe>
* os::pipe() is now unsafe because it does not arrange for deallocation of file
  descriptors
* os::Pipe fields are renamed from input to reader and out to write.
* PipeStream::pair() has been added. This is a safe method to get a pair of
  pipes.
* Dealing with pipes in native process bindings have been improved to be more
  robust in the face of failure and intermittent errors. This converts a few
  fail!() situations to Err situations.

Closes #9458
cc #13538
Closes #14724
[breaking-change]
auto merge of #14551 : alexcrichton/rust/issue-14467, r=kballard

Forking off a child which survives the parent is often a useful task, and is
currently not possible because the Process type will invoke `wait()` in its
destructor in order to prevent leaking resources. This function adds a new safe
method, `daemonize`, which can be used to consume an instance of `Process` which
will then not call `wait` in the destructor.

This new method is clearly documented as a leak of resources, but it must be
forcibly opted in to.

Closes #14467
auto merge of #14781 : alexcrichton/rust/issue-14724, r=brson

* os::pipe() now returns `IoResult<os::Pipe>`
* os::pipe() is now unsafe because it does not arrange for deallocation of file
  descriptors
* PipeStream::pair() has been added. This is a safe method to get a pair of
  pipes.
* Dealing with pipes in native process bindings have been improved to be more
  robust in the face of failure and intermittent errors. This converts a few
  fail!() situations to Err situations.

cc #13538
Closes #14724
[breaking-change]