Hide trait rc::RcBoxPtr from docs

It is for internal use only and should not appear in docs.
auto merge of #13705 : edwardw/rust/rcboxptr-doc, r=alexcrichton

It is for internal use only and should not appear in docs.
std: Add support for an accept() timeout

This adds experimental support for timeouts when accepting sockets through
`TcpAcceptor::accept`. This does not add a separate `accept_timeout` function,
but rather it adds a `set_timeout` function instead. This second function is
intended to be used as a hard deadline after which all accepts will never block
and fail immediately.

This idea was derived from Go's SetDeadline() methods. We do not currently have
a robust time abstraction in the standard library, so I opted to have the
argument be a relative time in millseconds into the future. I believe a more
appropriate argument type is an absolute time, but this concept does not exist
yet (this is also why the function is marked #[experimental]).

The native support is built on select(), similarly to connect_timeout(), and the
green support is based on channel select and a timer.

cc #13523
auto merge of #13688 : alexcrichton/rust/accept-timeout, r=brson

This adds experimental support for timeouts when accepting sockets through
`TcpAcceptor::accept`. This does not add a separate `accept_timeout` function,
but rather it adds a `set_timeout` function instead. This second function is
intended to be used as a hard deadline after which all accepts will never block
and fail immediately.

This idea was derived from Go's SetDeadline() methods. We do not currently have
a robust time abstraction in the standard library, so I opted to have the
argument be a relative time in millseconds into the future. I believe a more
appropriate argument type is an absolute time, but this concept does not exist
yet (this is also why the function is marked #[experimental]).

The native support is built on select(), similarly to connect_timeout(), and the
green support is based on channel select and a timer.

cc #13523
native: Remove unused and untested UnixDatagram
std: Add timeouts to unix connect/accept

This adds support for connecting to a unix socket with a timeout (a named pipe
on windows), and accepting a connection with a timeout. The goal is to bring
unix pipes/named sockets back in line with TCP support for timeouts.

Similarly to the TCP sockets, all methods are marked #[experimental] due to
uncertainty about the type of the timeout argument.

This internally involved a good bit of refactoring to share as much code as
possible between TCP servers and pipe servers, but the core implementation did
not change drastically as part of this commit.

cc #13523