Proposal: Add merge type 'interface' for types which implement an interface

[dionysius]

Adds a merge type Implements which sits between Type and Kind merge. Useful for cases where you have many types needed to merge in a specific way without the need to change the merge for a kind, since you don't want all of those kinds to be merged that way.

Personal real world example: Merge slices of pointers of some types in a specific way, but not all types of kind slice. E.g. Merge the elements of that slice by a specific Key (exposed by MergeKey() string or similiar), instead of just appending them.