Move alloc::prelude::* to alloc::prelude::v1, make alloc a subset of std

This was one of the unresolved questions of https://github.com/rust-lang/rfcs/pull/2480.
As the RFC says this is maybe not useful in the sense that we are unlikely
to ever have a second version, but making the crate a true subset
makes one less issue to think about if we stabilize it and later
want to merge standard library crates and have Cargo feature flags
to enable or disable parts of the `std` crate.

See also discussion in https://github.com/rust-lang/rust/pull/58175

src/liballoc/prelude/mod.rs

@@ -0,0 +1,15 @@
+//! The alloc Prelude
+//!
+//! The purpose of this module is to alleviate imports of commonly-used
+//! items of the `alloc` crate by adding a glob import to the top of modules:
+//!
+//! ```
+//! # #![allow(unused_imports)]
+//! # #![feature(alloc)]
+//! extern crate alloc;
+//! use alloc::prelude::v1::*;
+//! ```
+
+#![unstable(feature = "alloc", issue = "27783")]
+
+pub mod v1;

src/liballoc/prelude/v1.rs

@@ -1,14 +1,6 @@
-//! The alloc Prelude
+//! The first version of the prelude of `alloc` crate.
 //!
-//! The purpose of this module is to alleviate imports of commonly-used
-//! items of the `alloc` crate by adding a glob import to the top of modules:
-//!
-//! ```
-//! # #![allow(unused_imports)]
-//! # #![feature(alloc)]
-//! extern crate alloc;
-//! use alloc::prelude::*;
-//! ```
+//! See the [module-level documentation](../index.html) for more.
 
 #![unstable(feature = "alloc", issue = "27783")]