Add versions of [concat] that reduce judgmentally

Although proofs are more robust with the symmetric [concat], some proofs
are significantly faster or easier with a concat that reduces on one
side or the other.
Revert "Add versions of [concat] that reduce judgmentally"

This reverts commit 110f3c8c4a436a5f9a7d1b9c71f3c137c2970a3b.
WIP