blackbox algebra

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear Developers,

I have created a branch bill-bb_algebra which reimplement
RgX_RgXQ_eval (implementation of Brent and Kung algorithm)
as a generic function operating on a blackbox algebra, which
defines the following structure:

struct bb_algebra
{
  GEN (*red)(void *E, GEN x);
  GEN (*add)(void *E, GEN x, GEN y);
  GEN (*smul)(void *E, GEN x, GEN y);
  GEN (*mul)(void *E, GEN x, GEN y);
  GEN (*sqr)(void *E, GEN x);
  GEN (*one)(void *E);
  GEN (*zero)(void *E);
};

So far this is used only for RgX_RgXQ_eval and RgX_RgXQV_eval (and the
FpXQ/FlxqXQ/RgM version).

So this is rather experimental.

Cheers,
Bill.