rust-installer never verifies.

MacOS X does not ship with Java installed by default. Instead it
includes binary stubs that upon execution pop up a message suggesting
the installation of the JDK.

Since `javac` is only used when `antlr4` is available, it is possible
to work around the popup by only probing for `javac` if `antlr4` has
been successfully detected (in which case the JDK is probably already
installed on the system).

Fixes #23138.

This saves a bunch of a time and will make distributions smaller, as well as
avoiding filling the implementors page with internal garbage. Turn it back on
with `--enable-compiler-docs` if you want compiler docs during development.

Crates behind the facade are only documented on nightly/dev builds (where they
can be used).

[breaking-change]

Closes #23772
Closes #21297

The variable '$SHELL' is not actually defined by 'sh'.

LLVM older that 3.6 has a bug that cause assertions when compiling certain
constructs. For 3.5 there's still a chance that the bug might get fixed
in 3.5.2, so let's keep allowing to compile with it for it for now.

Not checking for 32/64 bit, since `uname -s` no longer contains an indicator (and `uname -m` returns correct results)

Using an extended regex fixes pattern matching on BSD sed.

Fixes #20467

When trying to build against a newer, local LLVM version it might be
preferable to have a flag to disable the LLVM version check instead of
having to modify the configure script.

Fixes #21998

the sed option `--in-place` (or `-i`) is a GNU extension, and it is not
portable to BSD system (openbsd and freebsd checked).

use an alternate construction in order to keep the semantic.

None of the others work, so let's remove them.

Fixes #17220.

Fixes #13179

Fixes #13082

This pulls all of our long-form documentation into a single document,
nicknamed "the book" and formally titled "The Rust Programming
Language."

A few things motivated this change:

* People knew of The Guide, but not the individual Guides. This merges
  them together, helping discoverability.
* You can get all of Rust's longform documentation in one place, which
  is nice.
* We now have rustbook in-tree, which can generate this kind of
  documentation. While its style is basic, the general idea is much
  better: a table of contents on the left-hand side.
* Rather than a almost 10,000-line guide.md, there are now smaller files
  per section.

This partially implements the feature staging described in the
[release channel RFC][rc]. It does not yet fully conform to the RFC as
written, but does accomplish its goals sufficiently for the 1.0 alpha
release.

It has three primary user-visible effects:

* On the nightly channel, use of unstable APIs generates a warning.
* On the beta channel, use of unstable APIs generates a warning.
* On the beta channel, use of feature gates generates a warning.

Code that does not trigger these warnings is considered 'stable',
modulo pre-1.0 bugs.

Disabling the warnings for unstable APIs continues to be done in the
existing (i.e. old) style, via `#[allow(...)]`, not that specified in
the RFC. I deem this marginally acceptable since any code that must do
this is not using the stable dialect of Rust.

Use of feature gates is itself gated with the new 'unstable_features'
lint, on nightly set to 'allow', and on beta 'warn'.

The attribute scheme used here corresponds to an older version of the
RFC, with the `#[staged_api]` crate attribute toggling the staging
behavior of the stability attributes, but the user impact is only
in-tree so I'm not concerned about having to make design changes later
(and I may ultimately prefer the scheme here after all, with the
`#[staged_api]` crate attribute).

Since the Rust codebase itself makes use of unstable features the
compiler and build system to a midly elaborate dance to allow it to
bootstrap while disobeying these lints (which would otherwise be
errors because Rust builds with `-D warnings`).

This patch includes one significant hack that causes a
regression. Because the `format_args!` macro emits calls to unstable
APIs it would trigger the lint.  I added a hack to the lint to make it
not trigger, but this in turn causes arguments to `println!` not to be
checked for feature gates. I don't presently understand macro
expansion well enough to fix. This is bug #20661.

Closes #16678

[rc]: https://github.com/rust-lang/rfcs/blob/master/text/0507-release-channels.md

Adds AArch64 knowledge to:
* configure,
* make files,
* sources,
* tests, and
* documentation.