Deprecate `Message::from_digest_slice`?

[Kixunil]

There is from_digest method which is more straightforward for probably all use cases - any serious hash engine will output an array, not a slice/vec/whatever. And in the rare scenarios where people use hash engines with crappy API they can just call .try_into().unwrap() themselves - no need to pollute our API.

[apoelstra]

Yeah, not a bad idea. Especially since we expect people to mostly stop using Message conversions anyway.