Use PPX instead of a custom DSL

[emillon]

let prog =
  let%bulletml act =
    repeat 500 @@ fire @@ repeat 10
      begin
        wait 60;
        speed 0;
        wait 60;
        speed 1;
      end;
    wait 10
  in
  [("top", act)]

[emillon]

While an EDSL would be nice:

• it doesn't have to be PPX. repeat can just be an (untyped) function.
• There needs to be a way to dynamically parse a program, for example for the JS demo.