Fixes #17103.

The implementation of LetSyntaxTT was specialized to macro_rules! in various
ways. This gets rid of the false generality and simplifies the code.

This commit moves the libserialize crate (and will force the hand of the
rustc-serialize crate) to not require the `old_orphan_check` feature gate as
well as using associated types wherever possible. Concretely, the following
changes were made:

* The error type of `Encoder` and `Decoder` is now an associated type, meaning
  that these traits have no type parameters.

* The `Encoder` and `Decoder` type parameters on the `Encodable` and `Decodable`
  traits have moved to the corresponding method of the trait. This movement
  alleviates the dependency on `old_orphan_check` but implies that
  implementations can no longer be specialized for the type of encoder/decoder
  being implemented.

Due to the trait definitions changing, this is a:

[breaking-change]

This fixes a mostly harmless syntax error in the install script.

This commit moves the libserialize crate (and will force the hand of the
rustc-serialize crate) to not require the `old_orphan_check` feature gate as
well as using associated types wherever possible. Concretely, the following
changes were made:

* The error type of `Encoder` and `Decoder` is now an associated type, meaning
  that these traits have no type parameters.

* The `Encoder` and `Decoder` type parameters on the `Encodable` and `Decodable`
  traits have moved to the corresponding method of the trait. This movement
  alleviates the dependency on `old_orphan_check` but implies that
  implementations can no longer be specialized for the type of encoder/decoder
  being implemented.

Due to the trait definitions changing, this is a:

[breaking-change]

cc #19260 

The casing transformations are left unstable (it is highly likely to be better to adopt the proper non-1-to-1 case mappings, per #20333) as are `is_xid_*`.

I've got a little todo list in the last commit of things I thought about/was told about that I haven't yet handled (I'd also like some feedback).

This commit introduces the syntax for negative implementations of traits
as shown below:

`impl !Trait for Type {}`

cc #13231
Part of RFC rust-lang/rfcs#127

r? @nikomatsakis 

This "reexports" all the functionality of `core::char::CharExt` as
methods on `unicode::u_char::UnicodeChar` (renamed to `CharExt`).

Imports may need to be updated (one now just imports
`unicode::CharExt`, or `std::char::CharExt` rather than two traits from
either), so this is a

[breaking-change]

Imports may need to be updated so this is a

[breaking-change]

TODOs:
  - ~~Entry is still `<'a, K, V>` instead of `<'a, O, V>`~~
  - ~~BTreeMap is still outstanding~~.
  - ~~Transform appropriate things into `.entry(...).get().or_else(|e| ...)`~~

Things that make me frowny face:
  - I'm not happy about the fact that this `clone`s the key even when it's already owned.
  - With small keys (e.g. `int`s), taking a reference seems wasteful.

r? @Gankro
cc: @cgaebel

This commit introduces the syntax for negative implmenetations of traits
as shown below:

`impl !Trait for Type {}`

cc #13231
Part of RFC #3

doc: Add missing `$`s in the Installing Rust guide

Reviewed-by: steveklabnik, steveklabnik

bench: remove warnings from rt-messaging-ping-pong.rs

Reviewed-by: alexcrichton

Allow rustdoc to accept vector pattern arguments

Reviewed-by: alexcrichton, alexcrichton

doc: remove incomplete sentence

Reviewed-by: steveklabnik, steveklabnik

Update guide index to point to the task page

Reviewed-by: alexcrichton

Fix range sugar

Reviewed-by: nick29581

Update guide for Cargo installation

Reviewed-by: steveklabnik

Fix typo in documentation.

Reviewed-by: alexcrichton

Man page/--help dialog fix

Reviewed-by: alexcrichton

Improve `make_hash` function

Reviewed-by: Gankro, Gankro

Remove graphviz::LabelText::* public reexport

Reviewed-by: cmr

Move rustup to the combined installer

Reviewed-by: brson

Put links to discuss.rust-lang.org and #rust-internals in CONTRIBUTING.m...

Reviewed-by: cmr

Fix vim syntax highlighting for `derive`

Reviewed-by: alexcrichton

Make all integers in the guide `i32`, implicitely

Reviewed-by: steveklabnik

doc: mailing list is deprecated

Reviewed-by: brson

Allow paths in constants to refer to polymorphic items.

Reviewed-by: nikomatsakis

This fixes an ICE that japaric was encountering in the wf checker.

r? @aturon