WIP-Improved documentation

* Improves documentation by re-arranging its structure and
  introducing full discussion about performance merit of
  intbitset.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Yes, but vectorial instruction set or not is a kind of "detail" that may be put to note, not to the ABOUT overview, to keep it short.  The numbers from the PERFORMANCE section speak about themselves...  (BTW instead of "will be exploited" may indicate this is not done yet...)