Simplify string-join using a loop

- The (if (null? result) ...) check can be done once, before the loop.
  It doesn't need to be done separately on each iteration.

- Carrying the result as a string, and using string-append on each
  iteration, is probably more efficient than gathering the strings into
  a temporary list. Avoiding apply will also avoid implementation limits
  on how many args can be passed to a procedure (if list is very long,
  it can exceed the limit).

- Use the names `lst` and `loop` as decided in #16.

recipes/join-list-of-strings-with-delimiter.md

@@ -13,15 +13,13 @@ SRFI 13 provides the [`string-join` procedure](https://srfi.schemers.org/srfi-13
 ### Using a loop
 
 ```
-(define (string-join list delimiter)
-  (let iter ((list list) (result '()))
-    (if (null? list)
-        (apply string-append (reverse result))
-        (let ((item (car list))
-              (rest (cdr list)))
-          (if (null? result)
-              (iter rest (cons item result))
-              (iter rest (cons item (cons delimiter result))))))))
+(define (string-join lst delimiter)
+  (if (null? lst) ""
+      (let loop ((result (car lst)) (lst (cdr lst)))
+        (if (null? lst)
+            result
+            (loop (string-append result delimiter (car lst))
+                  (cdr lst))))))
 ```
 
 Credit [Jakub T. Jankiewicz](https://jcubic.pl/me)