Optimise PPACK (#2067)

- pointers in `PPACK_IFACE_BLOCK` will be `__restrict__`
  - this is expected to produce better binary code for the _scalar_ case
- for GPU we expect this to better utilise data caches as per CUDA docs
- `ved_di` is not used anymore, gone and ABI bumped
- GPU code never used the indexing structs, so remove from
`PPACK_IFACE_BLOCK`