New function: qfbsolve

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

Hello PARI-dev,

I have added a new function qfbsolve.

qfbsolve(Q,p):

   Solve  the  equation  Q(x,y)  = p over the integers,  where Q is a imaginary
binary quadratic form and p a prime number.

   Return  [x,y]  as a two-components vector,  or zero if there is no solution.
Note that this functions return only one solution and not all the solutions.

This is a preliminary implementation. I plan to allow non prime p
and real binary quadratic.

This use a random polynomial time algorithm similar to cornacchia but
probably due to Gauss, using the reduction of quadratic form.

Cheers,
Bill.