Make SmallBox covariant over T

[PonasKovas]

Hello again.

I modified the implementation of SmallBox<T> to make it covariant over T, by replacing the inner pointer from *mut to *const.

It's worth noting that we have to be careful with the *const pointer, to not do something like &mut T as *const T, which results in a non-writable pointer (basically expands to &mut T as &T as *const T instead of &mut T as *mut T as *const T), but I didn't see anything like that in the codebase, as we are using the alloc crate which gives *mut u8 directly.