refactor(sterni/aoc/2021): fold over 2d array instead of nested list

Seems to save some allocations and thus recover some performance
compared to the two separate folds we had before.

Change-Id: Ie3d283103e6a9b8aa702db633d9c988fda1b2903
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4348
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/exercises/aoc/2021/solutions.bqn

@@ -239,7 +239,7 @@ day13Folds ← ParseFolds 1 ⊑ day13Input
 
 PerformAllFolds ← {𝕩 {(𝕨 _Fold)´𝕩}˜´ ⌽𝕨}
 DotMatrix ← {
-  ⟨width, height⟩ ← 1+⌈´𝕩
+  ⟨width, height⟩ ← 1+⌈˝∘‿2⥊∾𝕩
   {𝕩? '█';' '}¨ height‿width⥊≠¨⊔((⊣+(width⊸×)∘⊢)´)¨ 𝕩
 }