This commit stabilizes the following APIs:
* `TypeId::of` - now that it has an `Any` bound it's ready to be stable.
* `Box<Any>::downcast` - now that an inherent impl on `Box<Any>` as well as
`Box<Any+Send>` is allowed the `BoxAny` trait is removed in favor of these
inherent methods.
This is a breaking change due to the removal of the `BoxAny` trait, but
consumers can simply remove imports to fix crates.
[breaking-change]
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 = boxed::into_raw(b);
```
`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
