fflog: cubic sieve index calculus implementation

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

Dear developers,

I have commited a patch which implement the 'cubic sieve' index calculus algorithm
for fflog. This should allow to handle much larger fields.

An example:

a=ffgen(ffinit(37,13),'a);
b=ffprimroot(a);
fflog(a,b)

This takes 2s now while this would not finish in reasonable time with the previous
implementation.

The new algorithm is only available for field p^n with p>=3 and n>=5.
p=2 will be implemented separately.

Cheers,
Bill.