lib/string_exp: Introducing utf8 variant without indexes

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/string_exp/utf8_noindex: Added new constructor for UnicodeChar

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/string_exp/utf8_noindex: Added char_at, does the same as [] but with UnicodeChar.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/string_exp: Adapted String methods to UTF-8 string

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>