Clippy fix: borrow the HashMap Entry we want to insert into.

Clippy fix: borrow the HashMap Entry we want to insert into.

I hadn't known about this method of HashMap at all: you can test
whether a key exists, in such a way that you also get a reference to
the slot you'd want to insert it into if it didn't. Then you don't
need the two hash lookups you would with the more obvious approach of
separately testing for contains_key and then maybe inserting.