Add rustup.sh to the repo

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.
Improved examples on Vec documentation

- shrink_to_fit example is now more clear by asserting the capacity value
Changes a little the description of take in the guide

Closes #18218
regex: Fix control flow in the parser
regex: Escaped literals can end ranges
regex: The first range in a character class can start with a left bracket
Guide: Patterns: use non-x variables in match blocks
Guide: Closures: minor wording fixes
Guide: Closures fix suggested by @cakebaker
simplify shootout-reverse-complement.rs

Simpler, safer and shorter, in the same spirit of the current version, and the
same performances.
libcore: Inline atomic constructors.

Servo really wants this.
Fix some wording about errors

see https://github.com/rust-lang/rust/pull/18176#discussion_r19374679
return &mut T from the arenas, not &T

The arenas write the value to memory and then return a non-aliasing
reference to it. The returned reference can be mutable and can be
coerced to an immutable one.

[breaking-change]
Untabify rustup.sh
Add @thestinger comment explaining that shrink_to_fit might drop down as close as possible but not to the minimun
DSTify [T]/str extension traits

This PR changes the signature of several methods from `foo(self, ...)` to
`foo(&self, ...)`/`foo(&mut self, ...)`, but there is no breakage of the usage
of these methods due to the autoref nature of `method.call()`s. This PR also
removes the lifetime parameter from some traits (`Trait<'a>` -> `Trait`). These
changes break any use of the extension traits for generic programming, but
those traits are not meant to be used for generic programming in the first
place. In the whole rust distribution there was only one misuse of a extension
trait as a bound, which got corrected (the bound was unnecessary and got
removed) as part of this PR.

[breaking-change]
Small grammar fix in rustdoc.md

Happened to be reading through
Use PascalCase for token variants
Convert some token  functions into methods
Use an enum rather than a bool in token::Ident