In `core::RefMut::insert_unique`, used by `insert_entry` and others, we
were calling `reserve_entries` *before* the table insert, which defeats
the goal of matching capacities. We can't directly call that after table
insert though, because we'll be holding an `OccupiedEntry` that prevents
looking at the table itself. Instead, this code path now uses a more
typical doubling growth on the entries `Vec` itself, but still enhanced
by considering `MAX_ENTRIES_CAPACITY` as well.