changed mrb_str_size and mrb_str_bytesize comments.
Fix String#succ. "-".succ should be "."
Fix String#succ. "-a-".succ should be "-b-"
"-a-a-".succ should be "-a-b-"
Merge pull request #2523 from kkkkkt/changed_string_method_comment

changed mrb_str_size and mrb_str_bytesize comments.
Merge pull request #2524 from mattn/fix-succ

Fix String#succ. "-".succ should be "."