Speedup str[a:b] and PyUnicode_FromKindAndData

Speedup str[a:b] and PyUnicode_FromKindAndData

 * str[a:b] doesn't scan the string for the maximum character if the string
   is ascii only
 * PyUnicode_FromKindAndData() stops if we are sure that we cannot use a
   shorter character type. For example, _PyUnicode_FromUCS1() stops if we
   have at least one character in range U+0080-U+00FF