std::boxed::into_raw should be marked as safe

[shepmaster]

If I understand the reasoning behind marking mem::forget safe, then I think the same should apply to into_raw.

Currently, it says:

Function is unsafe, because result of this function is no longer automatically managed that may lead to memory or other resource leak.

[Gankra]

Agreed. It's safe. Basically a mem::forget that returns an integer.