Use mktemp for temporary download directory

Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.

The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
Fall back to hard coded download directory

If mktemp fails, fall back to a hard coded directory, per @nodakai's feedback.
Don't preserve line breaks in inline code

Closes #19323.
rustup: Don't do verbose tarball extraction

This creates an enormous amount of spew.
Added example to TempDir
Improve Arc<T> documentation, and Rc<T> docs a bit

Take the docs from Rc<T>, apply them to Arc<T>, and fix some line lengths.
Replaced wrapper functions with no_run and as_str().unwrap() with display()
Correct span in privacy error
doc: remove extraneous line
Use #[deriving(Copy)] for InvariantLifetime
Remove wrong `&str + String` and `&[T] + Vec<T>` implementations
doc: Small changes to ownership guide.

Disambiguate maximally by using 'and' instead of '&' next to discussion
about references.

As a bonus, fix the spelling of the car too.
remove l10n
Only count nested returns when the outer return is reachable

This narrows the definition of nested returns such that only when the
outer return has a chance of being executed (due to the inner return
being conditional) do we mark the function as having nested returns.

Fixes #19684
Add some documentation
Fix formatting issues
Minor fixes
Stability: tweak rules for trait impls

There is currently no way to specify the stability level for a trait
impl produced by `deriving`. This patch is a stopgap solution that:

* Turns of stability inheritance for trait impls, and
* Uses the stability level of the *trait* if no level is directly
  specified.

That is, manual trait impls may still provide a directly stability
level, but `deriving` will use the level of the trait. While not a
perfect solution, it should be good enough for 1.0 API stabilization, as
we will like *remove* any unwanted impls outright.
Fix logic error and add unreachable after returns
iOS: fallout of `marker::NoCopy` removal