These tests necessarily need to change now that `?Sized` is not
sufficient to accept extern types and `PointeeSized` is now necessary. In
addition, the `size_of_val`/`align_of_val` test can now be changed to
expect an error.
* ensuring that offset_of!(Self, ...) works iff inside an impl block
* ensuring that the output type is usize and doesn't coerce. this can be
changed in the future, but if it is done, it should be a conscious descision
* improving the privacy checking test
* ensuring that generics don't let you escape the unsized check
