RgXn class function for power series

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

Dear PARI developers,

I have added support for a new class of functions (RgXn)
for computation in Rg[X]/(X^n). This is useful to implement
operation on power series.

So far the following functions are available:

GEN     RgXn_eval(GEN Q, GEN x, long n);
GEN     RgXn_mul(GEN f, GEN g, long n);
GEN     RgXn_powers(GEN f, long m, long n);
GEN     RgXn_red_shallow(GEN a, long n);
GEN     RgXn_reverse(GEN f, long e);
GEN     RgXn_sqr(GEN f, long n);
GEN     RgXnV_red_shallow(GEN P, long n);
GEN     RgX_RgXn_eval(GEN Q, GEN x, long n);
GEN     RgX_RgXnV_eval(GEN Q, GEN x, long n);

See the documentation for detail.
(RgXn_reverse is an implementation of serreverse using Newton algorithm).

Cheers,
Bill.