new GP loop: forpart()

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

Dear PARI developers,

I have commited a patch by Pascal and myself that adds GP function
to loop over partitions:

forpart(v=5,print(v))
Vecsmall([1,1,1,1,1])
Vecsmall([1,1,1,2])
Vecsmall([1,2,2])
Vecsmall([1,1,3])
Vecsmall([2,3])
Vecsmall([1,4])
Vecsmall([5])

Also I have changed the partitions function to provide the same interface
as forpart. An unfortunate side effect is that the order of the partitions
is reversed. This could be fixed if someone provide a forpart_previous()
function.

Here the documentation:

forpart(X = k,seq,{a = k},{n = k}):

   Evaluate seq over the partitions X = [x_1,...x_n] of integer k,  that is increasing sequences
x_1 <= x_2... <= x_n of sum x_1+...x_n = k.

   Optional parameters n and a can be:

   *  n  = nmax  (resp.   n = [nmin,nmax])  to restrict partitions to have length less than nmax
(resp. length between nmin and nmax), where the length is the number of non zero entries.

   * a = amax  (resp.   a = [amin,amax])  to restrict the components of X to be to integers less
than amax (resp. between amin and amax).

   By  default,   remove  zero  entries  on the left except if amin <= 0,  in which case nmin is
ignored and X is of constant length nmax.

   Examples:

   \\ max 3 non-zero terms less than 4
   ? forpart(v=5,print(Vec(v)),3,4)
   [1, 2, 2]
   [1, 1, 3]
   [2, 3]
   [1, 4]

Cheers,
Bill