lib/string_exp/utf8_no_index: FlatBuffer is compatible with UTF-8

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
utf8_noindex/tests: Added a test for utf8 without index variant

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/string_exp/utf8_no_index: Updated README

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>