Fix crash in method call in `ObjectSpace.each_object` block.

* Filter `MRB_TT_ENV` and `MRB_TT_ICLASS` since it's internal object.
* Set `mrb->string_class` in `mrb_init_exception` instead.
 * Since https://github.com/mruby/mruby/commit/919af61766b1d06cd0e747c1d6257ada14c09eff isn't enough in this case.