Add string functions :

* string_it_get_pos to return the position in the steam
* string_it_pos to start iterating from a given position.

tests/test-mstring.c

@@ -84,6 +84,7 @@ static void test_utf8_it(void)
 
   string_init(s);
   string_it_end(it2, s);
+  assert( string_it_get_pos(it2) == 0);
 
   for(string_it(it, s) ; !string_end_p(it); string_next(it)) {
     assert(0); // Shall not be called
@@ -98,6 +99,13 @@ static void test_utf8_it(void)
     assert (i < 5);
     assert (string_get_cref(it) == tab[i]);
   }
+  assert(i == 5);
+  assert( string_it_get_pos(it) == 9);
+  string_it_pos(it2, s, 9);
+  assert( string_end_p(it2) );
+  string_it_pos(it2, s, 0);
+  assert( !string_end_p(it2) );
+  assert( string_get_cref(it2) == 45);
 
   string_set_str (s, "H€llo René Chaînôr¬");
   assert(string_length_u(s) == 19);