This reverts commit 604f049cd5.
This is purely a revert of cuviper's revert "Restore `T: Sized` on
`ptr::is_null`". So double revert means this is code written by cuviper!
The exact semantics of `is_null` on unsized pointers are still debatable,
especially for trait objects. It may be legal to call `*mut self`
methods on a trait object someday, as with Go interfaces, so `is_null`
might need to validate the vtable pointer too.
For `as_ref` and `as_mut`, we're assuming that you cannot have a non-null
data pointer with a null vtable, so casting the unsized check is fine.
`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.
