Remove `T: Sized` on `ptr::is_null()`, `as_ref()`, `as_mut()`

Remove `T: Sized` on `ptr::is_null()`, `as_ref()`, `as_mut()`

`NonZero::is_zero()` was already casting all pointers to thin `*mut u8`
to check for null.  It seems reasonable to apply that for `is_null()` in
general, and then unsized fat pointers can also be used with `as_ref()`
and `as_mut()` to get fat references.