Make Resolver protocol derivable with a default implementation for structs

[PragTob]

From José from the mailing list:

You can also look into making the protocol derivable with a default implementation that only merges structs of the same kind. This way someone can easily get deep merging for structs by adding "@derive DeepMerge.Resolver".

Got some more learning to do :)

[PragTob]

It's a merged on the master :)