hyperellcharpoly for y^2=x^(2*g+2)+...

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

Dear PARI developpers,

I have made an improvement to the GP function hyperellcharpoly()
so that now it also handles polynomials of even degrees.

? hyperellcharpoly((x^6-18*x^3-27)*Mod(1,7))
%1 = x^4-4*x^3+18*x^2-28*x+49

Cheers,
Bill.