Add a chunk procedure

Just a variant of chunk! which doesn't modify the provided list.

guix-data-service/utils.scm

@@ -31,6 +31,7 @@
             par-map&
             letpar&
 
+            chunk
             chunk!))
 
 (define (call-with-time-logging action thunk)
@@ -155,6 +156,16 @@
 
 (define par-map& (par-mapper' map cons))
 
+(define (chunk lst max-length)
+  (if (> (length lst)
+         max-length)
+      (call-with-values (lambda ()
+                          (split-at lst max-length))
+        (lambda (first-lst rest)
+          (cons first-lst
+                (chunk rest max-length))))
+      (list lst)))
+
 (define (chunk! lst max-length)
   (if (> (length lst)
          max-length)