syntactic_ast: modify the phantom types to have concrete constructors, as the ocaml compiler cannot prove them distinct otherwise

lib/compiler/syntactic_ast.mli

@@ -1,12 +1,12 @@
 type symbol = string
 
 (* These are just used for the GADT *)
-type expression
-type definition
-type clause (* for cond *)
-type binding (* for let *)
-type lambda_list
-type body
+type expression = Phantom_expr
+type definition = Phantom_def
+type clause = Phantom_clause (* for cond *)
+type binding = Phantom_binding(* for let *)
+type lambda_list = Phantom_lambda_list
+type body = Phantom_body
 
 type _ t =
   (* Literals *)