added an option to redefine control structures

tests/custom_control_structures.sl

@@ -0,0 +1,36 @@
+import stdlib/stdlib.sl
+import stdlib/control.sl
+
+word main
+    1 if
+        11 puti cr
+    else
+        99 puti cr
+    end
+
+    0 if
+        99 puti cr
+    else
+        22 puti cr
+    end
+
+    0 if
+        500 puti cr
+    else 1 if
+        33 puti cr
+    else
+        44 puti cr
+    end
+
+    0
+    5 for
+        1 +
+    end
+    puti cr
+
+    0
+    while dup 3 < do
+        1 +
+    end
+    puti cr
+end