1) #8271: when a byte sequence is invalid, only the start byte and all the
   valid continuation bytes are now replaced by U+FFFD, instead of replacing
   the number of bytes specified by the start byte.
   See http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf (pages 94-95);
2) 5- and 6-bytes-long UTF-8 sequences are now considered invalid (no changes
   in behavior);
3) Change the error messages "unexpected code byte" to "invalid start byte"
   and "invalid data" to "invalid continuation byte";
4) Add an extensive set of tests in test_unicode;
5) Fix test_codeccallbacks because it was failing after this change.