Link std::str to reference literals section
Add examples for all literal types in reference grouped together
auto merge of #18441 : mdinger/rust/literals, r=steveklabnik

Closes #18415

This links [`std::str`](http://doc.rust-lang.org/std/str/index.html) documentation to [literals](http://doc.rust-lang.org/reference.html#literals) in the reference guide and collects examples of literals into one group at the beginning of the section. ~~The new tables are not exhaustive (some escapes were skipped) and so I try to link back to the respective sections where more detail is located.~~ The tables are are mostly exhaustive. I misunderstood some of the whitespace codes.

I don't think the tables actually look that nice if that's important and I'm not sure how it could be improved. I think it does do a good job of collecting available options together. I think listing the escapes together is particularly helpful because they vary with type and are embedded in paragraphs.

[EDIT]
The [ascii table](http://man-ascii.com/) is here and may be useful.
Remove Runtime trait

This commit removes most of the remaining runtime infrastructure related
to the green/native split. In particular, it removes the `Runtime` trait
and instead inlines the native implementation.

Closes #17325

[breaking-change]
Remove libnative

With runtime removal complete, there's nothing left of libnative. This
commit removes it.

Fixes #18687

[breaking-change]
Remove libgreen

With runtime removal complete, there is no longer any reason to provide
libgreen.

[breaking-change]
Rewrite sync::mutex as thin layer over native mutexes

Previously, sync::mutex had to split between green and native runtime
systems and thus could not simply use the native mutex facility.

This commit rewrites sync::mutex to link directly to native mutexes; in
the future, the two will probably be coalesced into a single
module (once librustrt is pulled into libstd wholesale).
Fallout from libgreen and libnative removal
Make most of std::rt private

Previously, the entire runtime API surface was publicly exposed, but
that is neither necessary nor desirable. This commit hides most of the
module, using librustrt directly as needed. The arrangement will need to
be revisited when rustrt is pulled into std.

[breaking-change]
Fallout from new termination semantics
Fallout from namespaced enums
Loosen possibly bogus constraints in backtrace test
Disable dubious pipe test
auto merge of #18967 : aturon/rust/remove-runtime, r=alexcrichton

This PR completes the removal of the runtime system and green-threaded abstractions as part of implementing [RFC 230](https://github.com/rust-lang/rfcs/pull/230).

Specifically:

* It removes the `Runtime` trait, welding the scheduling infrastructure directly to native threads.

* It removes `libgreen` and `libnative` entirely.

* It rewrites `sync::mutex` as a trivial layer on top of native mutexes. Eventually, the two modules will be merged.

* It hides the vast majority of `std::rt`.

This completes the basic task of removing the runtime system (I/O and scheduling) and components that depend on it. 

After this lands, a follow-up PR will pull the `rustrt` crate back into `std`, turn `std::task` into `std::thread` (with API changes to go along with it), and completely cut out the remaining startup/teardown sequence. Other changes, including new [TLS](https://github.com/rust-lang/rfcs/pull/461) and synchronization are in the RFC or pre-RFC phase.

Closes #17325
Closes #18687

[breaking-change]

r? @alexcrichton
sys: reveal std::io representation to sys module

This commit adds a `AsInner` trait to `sys_common` and provides
implementations on many `std::io` types. This is a building block for
exposing platform-specific APIs that hook into `std::io` types.
libs: add std::os::unix module

The new `std::os::unix` module exposes several extension traits
for extracting file descriptors from `std::io` types.
libs: add std::os::windows module

The new `std::os::windows` module exposes several extension traits
for extracting file descriptors, sockets, and handles from `std::io`
types.