util: add es6 Symbol support for `util.inspect`

* `util.inspect` cannot accept es6 symbol primitive
* It will throw exception if do `util.inspect(Symbol())`
* This also affects repl, console.log, etc.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>