Sacrifice some micro-optimization to remove duplicate code
Fix crash in method call in `ObjectSpace.each_object` block.
* Filter `MRB_TT_ENV` and `MRB_TT_ICLASS`.
* Set `mrb->string_class` in `mrb_init_exception` instead.
ObjectSpace.each_object should filter out internal (i.e. obj->c == NULL) objects; ref #2429
remove String class initialization in error.c; ref #2429
git push origin masterMerge branch 'take-cheeze-fix_each_obj_crash'
Merge pull request #2430 from SHyx0rmZ/remove-duplicate

Sacrifice some micro-optimization to remove duplicate code