Fix memory leak in Ripper.sexp

rb_ast_dispose does not free the rb_ast_t causing it to be leaked. This
commit changes it to use rb_ast_free instead.

For example:

    require "ripper"

    10.times do
      100_000.times do
        Ripper.sexp("")
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    27648
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    62208
    67072
    71936

After:

    22784
    22784
    22784
    22784
    22912
    22912
    22912
    22912
    22912
    22912

parse.y

@@ -16076,7 +16076,7 @@ rb_ruby_ripper_parse0(rb_parser_t *p)
     parser_prepare(p);
     p->ast = rb_ast_new();
     ripper_yyparse((void*)p);
-    rb_ast_dispose(p->ast);
+    rb_ast_free(p->ast);
     p->ast = 0;
     p->eval_tree = 0;
     p->eval_tree_begin = 0;

test/ripper/test_ripper.rb

@@ -170,6 +170,14 @@ end
     end;
   end
 
+  def test_sexp_no_memory_leak
+    assert_no_memory_leak(%w(-rripper), "", "#{<<~'end;'}", rss: true)
+      1_000_000.times do
+        Ripper.sexp("")
+      end
+    end;
+  end
+
   class TestInput < self
     Input = Struct.new(:lines) do
       def gets