Provide a `into_raw` and `from_raw` API?

[FreeMasen]

I am curious if you would be willing to support a new pair of APIs similar to Box::into_raw/Box::from_raw?

To support for using this crate to back an Allocator, which is expected to provide/accept NonNull<[u8]>, it would be really nice to be able to convert either a Mmap or MmapMut into a NonNull<[u8]> or (MmapRawDescriptor, NonNull<[u8]>) which would disable the Drop implementation but allow for re-constructing from those parts to opt back into the Drop semantics.