BibSort: improved washers

* Improves speed and realiability of BibSort washers by properly
  exploiting textutils.decode_to_unicode() and
  textutils.translate_to_ascii() functions.

* Improves speed of textutils.translate_to_ascii().

* Makes unidecode dependency mandatory.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>