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.
Windows dbghelp strips leading underscores from symbols, so let's accept "ZN...E" form too.
Also, print PC displacement from symbols.
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.
Fix typo
Replaced wrapper functions with no_run and as_str().unwrap() with display()
Correct span in privacy error
doc: remove extraneous line
Add tests for E-needstest issues
Use #[deriving(Copy)] for InvariantLifetime
Remove wrong `&str + String` and `&[T] + Vec<T>` implementations
Only try to install the doc directory if it exists.

If you configure with `--disable-docs`, the `doc` directory does not get generated, so
`cp -r doc dist/` fails when you `make dist{,-tar-bins,-doc}` or `make install`
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