std: Internalize almost all of `std::rt`

This commit does some refactoring to make almost all of the `std::rt` private.
Specifically, the following items are no longer part of its API:

* DEFAULT_ERROR_CODE
* backtrace
* unwind
* args
* at_exit
* cleanup
* heap (this is just alloc::heap)
* min_stack
* util

The module is now tagged as `#[doc(hidden)]` as the only purpose it's serve is
an entry point for the `panic!` macro via the `begin_unwind` and
`begin_unwind_fmt` reexports.

destructors.md

@@ -26,13 +26,16 @@ this is totally fine.
 For instance, a custom implementation of `Box` might write `Drop` like this:
 
 ```rust
-#![feature(heap_api, core_intrinsics, unique)]
+#![feature(alloc, heap_api, core_intrinsics, unique)]
+
+extern crate alloc;
 
-use std::rt::heap;
 use std::ptr::Unique;
 use std::intrinsics::drop_in_place;
 use std::mem;
 
+use alloc::heap;
+
 struct Box<T>{ ptr: Unique<T> }
 
 impl<T> Drop for Box<T> {
@@ -45,6 +48,7 @@ impl<T> Drop for Box<T> {
         }
     }
 }
+# fn main() {}
 ```
 
 and this works fine because when Rust goes to drop the `ptr` field it just sees
@@ -54,13 +58,16 @@ use-after-free the `ptr` because when drop exits, it becomes inacessible.
 However this wouldn't work:
 
 ```rust
-#![feature(heap_api, core_intrinsics, unique)]
+#![feature(alloc, heap_api, core_intrinsics, unique)]
+
+extern crate alloc;
 
-use std::rt::heap;
 use std::ptr::Unique;
 use std::intrinsics::drop_in_place;
 use std::mem;
 
+use alloc::heap;
+
 struct Box<T>{ ptr: Unique<T> }
 
 impl<T> Drop for Box<T> {
@@ -87,6 +94,7 @@ impl<T> Drop for SuperBox<T> {
         }
     }
 }
+# fn main() {}
 ```
 
 After we deallocate the `box`'s ptr in SuperBox's destructor, Rust will
@@ -129,13 +137,16 @@ The classic safe solution to overriding recursive drop and allowing moving out
 of Self during `drop` is to use an Option:
 
 ```rust
-#![feature(heap_api, core_intrinsics, unique)]
+#![feature(alloc, heap_api, core_intrinsics, unique)]
+
+extern crate alloc;
 
-use std::rt::heap;
 use std::ptr::Unique;
 use std::intrinsics::drop_in_place;
 use std::mem;
 
+use alloc::heap;
+
 struct Box<T>{ ptr: Unique<T> }
 
 impl<T> Drop for Box<T> {
@@ -165,6 +176,7 @@ impl<T> Drop for SuperBox<T> {
         }
     }
 }
+# fn main() {}
 ```
 
 However this has fairly odd semantics: you're saying that a field that *should*

vec-alloc.md

@@ -9,7 +9,7 @@ This is perfectly fine because we already have `cap == 0` as our sentinel for no
 allocation. We don't even need to handle it specially in almost any code because
 we usually need to check if `cap > len` or `len > 0` anyway. The traditional
 Rust value to put here is `0x01`. The standard library actually exposes this
-as `std::rt::heap::EMPTY`. There are quite a few places where we'll
+as `alloc::heap::EMPTY`. There are quite a few places where we'll
 want to use `heap::EMPTY` because there's no real allocation to talk about but
 `null` would make the compiler do bad things.
 
@@ -20,11 +20,12 @@ the `heap` API anyway, so let's just get that dependency over with.
 So:
 
 ```rust,ignore
-#![feature(heap_api)]
+#![feature(alloc, heap_api)]
 
-use std::rt::heap::EMPTY;
 use std::mem;
 
+use alloc::heap::EMPTY;
+
 impl<T> Vec<T> {
     fn new() -> Self {
         assert!(mem::size_of::<T>() != 0, "We're not ready to handle ZSTs");

vec-final.md

@@ -2,17 +2,16 @@
 
 ```rust
 #![feature(unique)]
-#![feature(heap_api)]
+#![feature(alloc, heap_api)]
+
+extern crate alloc;
 
 use std::ptr::{Unique, self};
-use std::rt::heap;
 use std::mem;
 use std::ops::{Deref, DerefMut};
 use std::marker::PhantomData;
 
-
-
-
+use alloc::heap;
 
 struct RawVec<T> {
     ptr: Unique<T>,