Use mrb_str_cat_lit() instead of mrb_str_cat_cstr() for string-literals.
Merge pull request #2581 from cubicdaiya/issues/use_lit

Use mrb_str_cat_lit() instead of mrb_str_cat_cstr() for string-literals.
save and restore arena index to prevent arena overflow.
Merge pull request #2582 from iij/pr-attr-accessor-save-arena

save and restore arena index to prevent arena overflow.