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.
Add comments with type annotations.

This will hopefully help people with their first steps in Rust.

Fixes #16143.
Use rust-installer for installation

This is just a refactoring of the current installer so that Rust and Cargo
use the same codebase.

cc #16456
serialize: Avoid stray nul characters when auto-serializing char.

Fixes #19719.
Add coercions from *mut to *const and from &mut to *const.
rustc: Don't panic on corrupt metadata

Fix a panic where the compiler was looking at stale or old metadata.

See #19798, #19772, #19757, #19744, #19718, #19691.
Windows dbghelp strips leading underscores from symbols, so let's accept "ZN...E" form too.
Also, print PC displacement from symbols.
Deprecate more in-tree libs for crates.io

This commit deprecates a few more in-tree libs for their crates.io counterparts.
Note that this commit does not make use of the #[deprecated] tag to prevent
warnings from being generated for in-tree usage. Once #[unstable] warnings are
turned on then all external users will be warned to move.

These crates have all been duplicated in rust-lang/$crate repositories so
development can happen independently of the in-tree copies. We can explore at a
later date replacing the in-tree copies with the external copies, but at this
time the libraries have changed very little over the past few months so it's
unlikely for changes to be sent to both repos.

cc #19260
Change `VecMap`'s iterators to use wrapper structs instead of typedefs.

Using a type alias for iterator implementations is fragile since this
exposes the implementation to users of the iterator, and any changes
could break existing code.

This commit changes the iterators of `VecMap` to use
proper new types, rather than type aliases.  However, since it is
fair-game to treat a type-alias as the aliased type, this is a:

[breaking-change].
rustc: Start the deprecation of libserialize

The primary focus of Rust's stability story at 1.0 is the standard library.
All other libraries distributed with the Rust compiler are planned to
be #[unstable] and therfore only accessible on the nightly channel of Rust. One
of the more widely used libraries today is libserialize, Rust's current solution
for encoding and decoding types.

The current libserialize library, however, has a number of drawbacks:

* The API is not ready to be stabilize as-is and we will likely not have enough
  resources to stabilize the API for 1.0.
* The library is not necessarily the speediest implementations with alternatives
  being developed out-of-tree (e.g. serde from erickt).
* It is not clear how the API of Encodable/Decodable can evolve over time while
  maintaining backwards compatibility.

One of the major pros to the current libserialize, however, is
`deriving(Encodable, Decodable)` as short-hands for enabling serializing and
deserializing a type. This is unambiguously useful functionality, so we cannot
simply deprecate the in-tree libserialize in favor of an external crates.io
implementation.

For these reasons, this commit starts off a stability story for libserialize by
following these steps:

1. The deriving(Encodable, Decodable) modes will be deprecated in favor of a
   renamed deriving(RustcEncodable, RustcDecodable).
2. The in-tree libserialize will be deprecated in favor of an external
   rustc-serialize crate shipped on crates.io. The contents of the crate will be
   the same for now (but they can evolve separately).
3. At 1.0 serialization will be performed through
   deriving(RustcEncodable, RustcDecodable) and the rustc-serialize crate. The
   expansions for each deriving mode will change from `::serialize::foo` to
   `::rustc_serialize::foo`.

This story will require that the compiler freezes its implementation of
`RustcEncodable` deriving for all of time, but this should be a fairly minimal
maintenance burden. Otherwise the crate in crates.io must always maintain the
exact definition of its traits, but the implementation of json, for example, can
continue to evolve in the semver-sense.

The major goal for this stabilization effort is to pave the road for a new
official serialization crate which can replace the current one, solving many of
its downsides in the process. We are not assuming that this will exist for 1.0,
hence the above measures. Some possibilities for replacing libserialize include:

* If plugins have a stable API, then any crate can provide a custom `deriving`
  mode (will require some compiler work). This means that any new serialization
  crate can provide its own `deriving` with its own backing
  implementation, entirely obsoleting the current libserialize and fully
  replacing it.

* Erick is exploring the possibility of code generation via preprocessing Rust
  source files in the near term until plugins are stable. This strategy would
  provide the same ergonomic benefit that `deriving` does today in theory.

So, in summary, the current libserialize crate is being deprecated in favor of
the crates.io-based rustc-serialize crate where the `deriving` modes are
appropriately renamed. This opens up space for a later implementation of
serialization in a more official capacity while allowing alternative
implementations to be explored in the meantime.

Concretely speaking, this change adds support for the `RustcEncodable` and
`RustcDecodable` deriving modes. After a snapshot is made warnings will be
turned on for usage of `Encodable` and `Decodable` as well as deprecating the
in-tree libserialize crate to encurage users to use rustc-serialize instead.
Fix mispelling in char.rs error message

Error message has wrong spelling ("radix is to high").
libsyntax: Make deriving also respect where bounds.
libsyntax: Output where clauses in pretty printer for structs.
Modify `regex::Captures::{at,name}` to return `Option`

Closes #14602.  As discussed in that issue, the existing `at` and `name`
functions represent two different results with the empty string:

1. Matched the empty string.
2. Did not match anything.

Consider the following example.  This regex has two named matched
groups, `key` and `value`. `value` is optional:

```rust
// Matches "foo", "foo;v=bar" and "foo;v=".
regex!(r"(?P<key>[a-z]+)(;v=(?P<value>[a-z]*))?");
```

We can access `value` using `caps.name("value")`, but there's no way for
us to distinguish between the `"foo"` and `"foo;v="` cases.

Early this year, @BurntSushi recommended modifying the existing `at` and
`name` functions to return `Option`, instead of adding new functions to
the API.

This is a [breaking-change], but the fix is easy:

- `refs.at(1)` becomes `refs.at(1).unwrap_or("")`.
- `refs.name(name)` becomes `refs.name(name).unwrap_or("")`.
InvariantLifetime is Copy-able

Both ContravariantLifetime and CovariantLifetime are marked as Copy,
so it makes sense for InvariantLifetime to be as well.
libcore: make iterator adaptors `Clone`able
Fix indentation in a code example in the ownership guide.
Fix typo: intuitive -> unintuitive