internal/core/adt: add benchmark for 3881

Issue #3881

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I8dea8779b3fe23eaaea688f72a62749928eeb4db
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1213606
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>

Author: mpvl