Refactor: alleviate painful AsHashDB impls with macro (or magic)

[dvdplm]

In the course of Make HashDB generic changes were made to the HashDB trait that made it impossible to keep the generic impls of AsHashDB we had before. To work around this concrete impls for AsHashDB were added in many places. Find a more elegant way to deal with the issue, e.g. by adding a impl_as_hash_db! macro.

See the NOTE in the code for more details.