update day5 to use terse array syntax

array.(index) <- some_value

src/day5.ml

@@ -1,14 +1,11 @@
 open Containers
 
-let crane_drop stacks index box =
+let crane_drop stacks index boxes = stacks.(index) <- boxes @ stacks.(index)
-  let stack = Array.get stacks index in
-  Array.set stacks index (box @ stack)
-;;
 
 let crane_lift stacks index count =
-  let stack = Array.get stacks index in
+  let stack = stacks.(index) in
   let boxes, rest = List.take_drop count stack in
-  Array.set stacks index rest;
+  stacks.(index) <- rest;
   boxes
 ;;