Auto merge of #6218 - korrat:master, r=ebroto

Auto merge of #6218 - korrat:master, r=ebroto

Add lint for maps with zero-sized value types

Hi, this is my first time contributing to clippy or rust in general, so I'm not sure about the details of contributing. Please excuse me and let me now if I did anything wrong. I have a couple of questions:

1. I'm not sure what category this lint should be. I've put it in "nursery" for now.
1. Should I squash commits this is reviewed/merged?

changelog: Add lint for maps with zero-sized value types

Fixes #1641