Style
bench conversion
slight speedup in string_to_vec_of_utf8char

test string_to_vec_of_char           ... bench:       965 ns/iter (+/- 50)
test string_to_vec_of_utf8char       ... bench:       740 ns/iter (+/- 70)
Implement DoubleEndedIterator for Utf8Chars
Use DoubleEndedIterator