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().
  (addresses #1754)

* Introduces new washer
  _sort_nosymbols_case_insensitive_strip_accents().

* Makes unidecode dependency mandatory.
  (PR #2283)

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