Refactor DigestItem (#2108)

* Add `start_aura2`.

* .gitignore patch conflict files

and remove one that accidentally got committed

* Fix build

The tests still don’t work.

* Fix compilation errors

* Fix compile errors (again)

* Try (and fail) to fix tests

* Properly deserialize data

Previously, `DigestItem::Consensus` had no separate `DigestItemType`,
so it did not get properly serialized and deserialized.

* Add extra debug logging.  Always allow old seals.

A `RUST_LOG=substrate_aura_consensus cargo test --all -- --nocapture \
tests::authoring_blocks` revealed that old seals were being and
rejected, causing the test to hang.  As a temporary debug measure, allow
old seals unconditionally, so that CI can test if this fixes the
problem.

* Forcibly disable rejection of old seals

* Use old trait, but newer serialization

The old trait for `CompatibleDigestItem` actually worked.  By changing
its implementation, one can ensure that all *new* seals have the modern
f...