Use from_raw and into_raw instead of transmuting the box for callback ownership

[yupferris]

I saw this forum post today, and it's mentioned that using Box's from_raw and into_raw to go to/from a c-pointer is preferred to using transmute. I'm guessing if nothing else this is to make it more explicit what's going on.

[yupferris]

There would be no need to repackage/publish with this change either, as it won't change any behavior in the crate; it's simply a cosmetic improvement.

[yupferris]

This is fixed on my remove-transmute branch, but into_raw and from_raw aren't actually stable, so we can't merge this change yet.

[yupferris]

These methods are stable in beta however, so they should be available in 2 weeks or so.