Use var List instead of ListBuffer to save memory (#2465) (#2467)

This reduces memory use of every HasId by 64 bytes.

Every instance of HasId (including all Data) had 2 ListBuffer vals for
recording post-naming hooks, yet this feature is almost never used.
These are now vars of type List which allows the common case of Nil to
add no incremental memory use per instance of HasId.

(cherry picked from commit cf410180

)

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