Work in progress: add char_utf8bytes (#493)

* Add char_utf8bytes to library

* Improved implementation

* Improve code + add chars predicate

* Update example

* Renamed string_utf8bytes to char_utf8bytes

src/prolog/examples/utf8.pl

@@ -0,0 +1,15 @@
+:- use_module(library(charsio)).
+:- use_module(library(lists)).
+
+:- initialization(unit_test).
+
+unit_test :-
+  chars_utf8bytes("a£\x2124\", Bs),
+  Bs = [97, 194, 163, 226, 132, 164].
+
+write_f :-
+  open('x.txt', write, Stream, [type(binary)]),
+  F = put_byte(Stream),
+  chars_utf8bytes("£\x2124\\x2764\\x1F496\\n", Bs),
+  maplist(F, Bs),
+  close(Stream).