Trac #15601: Implementation of power series using PARI

Add a class `PowerSeries_pari` for power series based on PARI's `t_SER`
type.  At least if the base ring is a finite field, this is much faster
than the existing implementation based on NTL polynomials.

URL: https://trac.sagemath.org/15601
Reported by: pbruin
Ticket author(s): Peter Bruin
Reviewer(s): Travis Scrimshaw

Author: Release Manager

src/doc/en/reference/power_series/index.rst

@@ -7,6 +7,7 @@ Power Series Rings
    sage/rings/power_series_ring
    sage/rings/power_series_ring_element
    sage/rings/power_series_poly
+   sage/rings/power_series_pari
 
    sage/rings/multi_power_series_ring
    sage/rings/multi_power_series_ring_element

src/module_list.py

@@ -1275,6 +1275,9 @@ def uname_specific(name, value, alternative):
     Extension('sage.rings.power_series_poly',
               sources = ['sage/rings/power_series_poly.pyx']),
 
+    Extension('sage.rings.power_series_pari',
+              sources = ['sage/rings/power_series_pari.pyx']),
+
     Extension('sage.rings.power_series_ring_element',
               sources = ['sage/rings/power_series_ring_element.pyx']),
 

src/sage/rings/power_series_pari.pxd

@@ -0,0 +1,5 @@
+from sage.libs.cypari2.gen cimport Gen as pari_gen
+from power_series_ring_element cimport PowerSeries
+
+cdef class PowerSeries_pari(PowerSeries):
+    cdef pari_gen g