Similar to f3206cc79b but for TypedData.
It's quite common for TypedData objects to have a mix of reference in
their struct and some ivars.
Since we do happen to have 8B free in the RtypedData struct, we could
use it to keep a direct reference to the IMEMO/fields saving having
to synchronize the VM and lookup the `gen_fields_tbl` on every ivar
access.
For old school Data classes however, we don't have free space, but
this API is soft-deprecated and no longer very common.