lib/standard/string: to_upper/lower usable with UTF-8

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
lib/standard/string: output adapted for UTF-8

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
to_cstring bugfix
UTF8 code_point fix

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
header fix
unicode char new fix
UTFChar to_s fix
to_upper/lower fix
make_index fix
new NativeNitChar fix
Char view prototype FlatString
tests: Test for UTF-8 Strings.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Fix len to support invalid UTF8 chars.
fix substring