Optimize memory use of naming prefixes (#2471) (#2480)

* Use a single field instead of two in HasId (4-bytes per HasId)
* Set the prefix to Nil after setting ref to free up memory

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 3aa179f0

)

Co-authored-by: Jack Koenig <koenig@sifive.com>