`Base.merge` method for `NoMetadata`

[Beforerr]

Since NoMetadata is kinda of like an AbstractDict, maybe we could define something like merge to let merge works. Maybe we could also add merge for AbstractMetadata.

Other methods that may be of interest: empty.

[rafaqz]

Those methods would be useful

[tiemvanderdeure]

This would be a way to handle things like broadcasting where we could return merged metadata.

Needs some thought though, because metadata can wrap dicts but I think sometimes also NamedTuple, so we need to make sure all the dispatches are unambiguous and make sense.