Add sourcemap generator into compile procedure.

Now you can have original source position in stack trace.

```gorillascript
let x()
  throw new Error("hello")

x()
```

```log
Error: hello
  at x (/home/unco/snips/gerror/error.gs:2:8, <js>:4:11)
  at Object.<anonymous> (/home/unco/snips/gerror/error.gs:4:1, <js>:6:3)
  at Object.<anonymous> (/home/unco/snips/gerror/error.gs <js>:7:3)
  at Module._compile (module.js:456:26)
  at _step (/home/unco/repos/new/gorillascript/lib/gorilla.js:1690:31)
  at Object._send [as send] (/home/unco/repos/new/gorillascript/lib/gorilla.js:1710:16)
  at continuer (/home/unco/repos/new/gorillascript/lib/gorilla.js:189:31)
  at callback (/home/unco/repos/new/gorillascript/lib/gorilla.js:200:14)
  at Array.step [as 0] (/home/unco/repos/new/gorillascript/lib/gorilla.js:53:28)
  at Object._onImmediate (/home/unco/repos/new/gorillascript/lib/gorilla.js:26:25)
```

BTW, the sourcemap generated by gorillascript is not correct in the first trace frame.