Handle the for…end syntax

lib/syntax/S.ml

@@ -90,6 +90,15 @@ module type Instruction = sig
     T.assignation_operator ->
     expression ->
     t
+
+  val for_ :
+    pos ->
+    (pos, expression) variable ->
+    start:expression ->
+    to_:expression ->
+    step:expression option ->
+    t list ->
+    t
 end
 
 module type Location = sig