Use RARRAY_* macro instead of accessing RArray field.
Add tests for utf8-string#rindex
Merge pull request #2077 from take-cheeze/rstruct_macro

Use RARRAY_* macro instead of accessing RArray field.
Merge pull request #2079 from suzukaze/add-test-of-utf8-string.rindex

Add tests for String#rindex of mruby-string-utf8.