Add relay parent to inherent and to digest

[JoshOrndorff]

This is my attempt at solving paritytech/polkadot-sdk#83.

Feedback and suggestions welcome.

[bkchr]

Ty for starting this!

So, I imagined it a little bit different. The digest is put as pre-runtime digest to the block in the collator implementation here:

cumulus/collator/src/lib.rs

Line 506 in b5f6580

Default::default(),

We would also need to check here that the pre-runtime digest exists:
https://github.com/paritytech/cumulus/blob/master/consensus/src/import_queue.rs

Regarding interfaces I would like to see something similar to this here:
https://github.com/paritytech/substrate/blob/master/primitives/consensus/babe/src/digests.rs#L158-L213

[JoshOrndorff]

I might return to this, but I'm setting it aside for now. Thanks for the feedback on my initial draft.