A new module: bitset

[Akim Demaille]

Dead gnulibers,

In Bison, the core algorithms of the computation of the parsing tables
heavily use bitsets and arrays of bitsets.  Initially Bison used its
own routines for bitset manipulation (actually, a lot was done by
hand, without any abstraction), but at some point I saw a very nice
submission from Michael Hayes on the GCC lists for a fully featured
lib on bitsets.  (https://gcc.gnu.org/ml/gcc/2002-01/msg00825.html).

I don't know whether it finally made it in GCC
(https://gcc.gnu.org/ml/gcc/2002-05/msg01564.html), but it was
included in Bison, and we maintained it over the years (mostly
modernizing).

I think it should move to gnulib: it's way more powerful than what
we use in Bison only.  Paul already reported he would be interested
in using this module in Emacs.

This library is very flexible and implements several backends,
depending on the expected use (whether dense/sparse, fixed/dynamic,
etc.).  The headers are largely commented; but I have written a short
doc so that the reader can get at least a sense of how to use this
library.

Cheers!