new digits() function

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

Dear PARI developers,

I have added a GP function digits(n,B) to compute the digits in base B of n.
? digits(167,3)
%7 = [2,0,0,1,2]

Together with Pol, it can be used to convert integers to polynomials:
? Pol(digits(167,3))
%1 = 2*x^4+x+2
The reverse can be done using subst:
? subst(%1,x,3)
%2 = 167

Cheers,
Bill.