[lint request] unnecessary_sized_box_transmute

People transmute raw pointers, or... references (yeah, really), into Box<T: Sized>s...

We should catch these, telling people to properly casts their pointers and/or references and to use Box::from_raw and Box::into_raw methods (note: we probably want a different lint for T: ?Sized since trait objects have a different layout, and right now there isn't really a stable way to do that, so this might not be worth warning for now).

If this happens inside a macro, I don't know if we should still warn. The only case in which this could ever be wrong is if Box<T, A: Alloc> is monomorphized with a stateful allocator (non-ZST), but the transmute should catch that because the size would differ, so... maybe if this happens inside a macro, we should just not lint it (who knows what else the macro might be transmuting).

No files, tests, or entry points are named in the issue. Start by resolving the proposed lint's scope, including T: Sized, macro expansion, and stateful allocators; done means raw-pointer or reference transmutes into Box<T> are diagnosed with guidance toward proper casts and Box::from_raw/Box::into_raw.