std: Stabilize the prelude module

This commit is an implementation of [RFC 503][rfc] which is a stabilization
story for the prelude. Most of the RFC was directly applied, removing reexports.
Some reexports are kept around, however:

* `range` remains until range syntax has landed to reduce churn.
* `Path` and `GenericPath` remain until path reform lands. This is done to
  prevent many imports of `GenericPath` which will soon be removed.
* All `io` traits remain until I/O reform lands so imports can be rewritten all
  at once to `std::io::prelude::*`.

This is a breaking change because many prelude reexports have been removed, and
the RFC can be consulted for the exact list of removed reexports, as well as to
find the locations of where to import them.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0503-prelude-stabilization.md
[breaking-change]

Closes #20068

src/libsyntax/ptr.rs

@@ -10,16 +10,18 @@
 
 //! The AST pointer
 //!
-//! Provides `P<T>`, a frozen owned smart pointer, as a replacement for `@T` in the AST.
+//! Provides `P<T>`, a frozen owned smart pointer, as a replacement for `@T` in
+//! the AST.
 //!
 //! # Motivations and benefits
 //!
-//! * **Identity**: sharing AST nodes is problematic for the various analysis passes
-//!   (e.g. one may be able to bypass the borrow checker with a shared `ExprAddrOf`
-//!   node taking a mutable borrow). The only reason `@T` in the AST hasn't caused
-//!   issues is because of inefficient folding passes which would always deduplicate
-//!   any such shared nodes. Even if the AST were to switch to an arena, this would
-//!   still hold, i.e. it couldn't use `&'a T`, but rather a wrapper like `P<'a, T>`.
+//! * **Identity**: sharing AST nodes is problematic for the various analysis
+//!   passes (e.g. one may be able to bypass the borrow checker with a shared
+//!   `ExprAddrOf` node taking a mutable borrow). The only reason `@T` in the
+//!   AST hasn't caused issues is because of inefficient folding passes which
+//!   would always deduplicate any such shared nodes. Even if the AST were to
+//!   switch to an arena, this would still hold, i.e. it couldn't use `&'a T`,
+//!   but rather a wrapper like `P<'a, T>`.
 //!
 //! * **Immutability**: `P<T>` disallows mutating its inner `T`, unlike `Box<T>`
 //!   (unless it contains an `Unsafe` interior, but that may be denied later).
@@ -34,9 +36,9 @@
 //!   implementation changes (using a special thread-local heap, for example).
 //!   Moreover, a switch to, e.g. `P<'a, T>` would be easy and mostly automated.
 
-use std::fmt;
-use std::fmt::Show;
+use std::fmt::{mod, Show};
 use std::hash::Hash;
+use std::ops::Deref;
 use std::ptr;
 use serialize::{Encodable, Decodable, Encoder, Decoder};