add join function to stdlib

Signed-off-by: Ava Affine <ava@sunnypup.io>

snippets/userlib.f

@@ -146,3 +146,13 @@ adds a directory to PATH'
             (rem (cdr path-iter)))
         (echo pat)
         (set (q path-iter) (pop rem))))))
+
+(def join
+'concatenates list elements into a string, while
+interspersing a provided delimiter in between elements'
+  (list delim)
+  (reduce (lambda (res elem)
+            (if (eq? (strlen res) 0)
+              (string elem) 
+              (concat res delim elem)))
+    list))