Relax types on DigestItemRef, such that byte slices can be used in addition to vector references

[nazar-pc]

The goal here is to use DigestItemRef with frame_support::traits::FindAuthor, so I can turn (ConsensusEngineId, &'a [u8]) into DigestItemRef without extra allocation:

pub trait FindAuthor<Author> {
	/// Find the author of a block based on the pre-runtime digests.
	fn find_author<'a, I>(digests: I) -> Option<Author>
	where
		I: 'a + IntoIterator<Item = (ConsensusEngineId, &'a [u8])>;
}

Ideally, I'd like to see FindAuthor having &'a DigestItem instead of (ConsensusEngineId, &'a [u8]) if that is OK (this PR is still useful though).

On a similar note DigestItemRef should really use ConsensusEngineId instead of &'a ConsensusEngineId because reference will be more expensive to copy than the data itself.

Technically this PR introduces a breaking change to the API, but it is extremely unlikely to be an issue in practice as the source of that iterator is almost certainly Vec<DigestItem>.

[bkchr]

Looks good, beside the version bump.

[bkchr]

bot merge

[paritytech-processbot]

Waiting for commit status.

[paritytech-processbot]

Merge cancelled due to error. Error: Checks failed for f3fe72f

[bkchr]

@nazar-pc Can you add a polkadot companion?

[nazar-pc]

This PR is meant to be fully compatible with it and indeed I see no issues locally 🤷‍♂️
CI errors do not seem related to this PR, I pulled master again just in case.

[bkchr]

This didn't helped.

[nazar-pc]

I think there is something related to lock file that forces it to load incorrect version of the package whose version was updated, but I can't reproduce the issue locally.

[bkchr]

Can you merge master again. I will then merge this pr manually.