Don't check for the maximum character when copying from unicodeobject.c

* Create copy_characters() function which doesn't check for the maximum
   character in release mode
 * _PyUnicode_CheckConsistency() is no more static to be able to use it
   in _PyUnicode_FormatAdvanced() (in formatter_unicode.c)
 * _PyUnicode_CheckConsistency() checks the string hash