Auto merge of #21318 - stepancheg:box-fns, r=alexcrichton

Functions are needed for safety and convenience.

It is a common pattern to use `mem::transmute` to convert between
`Box` and raw pointer, like this:

```
let b = Box::new(3);
let p = mem::transmute(b);
// pass `p` to some C library
```

After this commit, conversion can be written as:

```
let p = b.into_raw();
```

`into_raw` and `from_raw` functions are still unsafe, but they are
much safer than `mem::transmute`, because *raw functions do not
convert between incompatible pointers. For example, this likely
incorrect code can be successfully compiled:

```
let p: *mut u64 = ...
let b: Box<u32> = mem::transmute(p);
```

Using `from_raw` results in compile-time error:

```
let p: *mut u64 = ...
let b: Box<u32> = Box::from_raw(p); // compile-time error
```

`into_raw` and `from_raw` functions are similar to C++ `std::unique_ptr`
`release` function [1] and constructor from pointer [2].

[1] http://en.cppreference.com/w/cpp/memory/unique_ptr/release
[2] http://en.cppreference.com/w/cpp/memory/unique_ptr/unique_ptr

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