Combine lemmas that are the same

[verdude]

Add the senses and attributions from one of them to the other when the lemma and part of speech are the same

[gliese1337]

It would probably be best to implement a generic lemma-combining function- something that takes two lemma structures as input and returns a single combined one- that can be re-used in multiple services.
A single generic "can these be combined" function would also be useful.

Ideally, we might want a single generic function that takes a list of lemmas and automatically figures out which ones can be combined and then does so, so that we don't have to reinvent that logic for every service, either.