Fix CandidateHash newtype

[eskimor]

In general Hash is way to overloaded in the codebase, CandidateHash was a glorious exception, being a newtype around Hash, but due to this DeRef instance, it is still interchangeable, limiting the added type safety.

We should:

1. Remove the CandidateHash DeRef instance.
2. Introduce more proper newtype wrappers. E.g. for the head data hash of a parachain, also relay block hashes should not be plain Hash ... although that change will be massive and probably overkill if we at least use newtypes for all other hashes.

[programskillforverification]

We can use macro to wrapper newtype around a [Hash] type. Not only for CandidateHash, but also all defined hashtype. I'd like to work on this issue, but maybe that's a breaking change.

[eagr]

Stuff whose hash I could think of that might use a newtype (besides relay chain block)

• code
• collation
• proof-of-validity
• storage root
• erasure coding root

But I'm not sure coz I don't see the entire picture. Anything you'd like to add? :) @eskimor