syntactic_ast: fix issue in set! parsing, add set! to the test case.

bin/comp.ml

@@ -20,7 +20,7 @@ and dbg_print_start = function
 
 let def = Parser.parse_str "(define (f)
                              (let ((x 5))
-                              (if t (+ x 1))))
+                              (if t (set! x (+ x 1)))))
                             (define (f)
                               (define (g y) (* y 2))
                               (or (g 5) (g 6)))

lib/compiler/syntactic_ast.ml

@@ -204,6 +204,7 @@ and builtin_if cons =
   exp (If (test, then_branch, else_branch))
 
 and builtin_set cons =
+  let* cons = sexpr_cdr cons in
   let* sym = sexpr_car cons in
   let* sym = (match sym with
              | LSymbol s -> Ok s