This exposes the parts of the reassembly buffer to be usable as context
as proposed in #8511.

I only exposed *parts of* for two reasons:

1. I don't need to expose further types (like `rbuf_int_t`), that are
   not of interest outside of fragmentation.
2. This allows for an easy future extension for the virtual reassembly
   buffer as proposed in [[1]].

This makes this change a little bit more involved, because instead of
just renaming the type, I also need to add the usage of the `super`
member, but I think in the end this little preparation work will be
beneficial in the future.

[1]: https://tools.ietf.org/html/draft-watteyne-6lo-minimal-fragment-01#section-3