Implement BoxedRep proposal
This is a rebase of !2249 (closed) on top of !2952 (merged) implementing GHC proposal #203.
In my local testing this does regress a bit relative to !2952 (merged) but there are no regressions relative to master
. Depends upon !4823 (closed).
Closes #17526 (closed).
Attributions
This was originally authored by @andrewthad, @treeowl did must of a significant rebase which was later finished by @andrewthad. I then did a final rebase.