Casting `*const T` to `*const U` with `U: Sized` is allowed even if `T: ?Sized`.
This safely extracts the data pointer out of a trait object,
without relying on the memory representation of trait objects.
This commit splits the style and layout data in two separate refcells.
These transmutes have been a source of trouble (for example on Android), and
they feel like a hack anyway.
Fixes#16982
