Two flavors of NonNull (again)

[Jules-Bertholet]

const and mut is not really sufficient, we really should have a third type for pointers to owned memory:

`*move` raw pointers

Over the years, there has been ample discussion on the concept of a "&move"/"&own"/"move reference"/"owning reference". However, its raw pointer cousin has been comparatively neglected. This topic aims to correct that shortfall. For & (reference to value you don't own and can't modify), we have *const. For &mut (reference to value you don't own but can modify), we have *mut. But for Box, Vec, Arc, and all the other safe-code references to values you own… we have, erm, NonNull? Except that's n…