Implement modular exponentiation for all polynomial rings

[jpflori]

sage: x = polygen(ZZ)
sage: pow(x,100,x+1)
x^100

CC: @roed314 @jdemeyer @pjbruin @sagetrac-jakobkroeker

Component: algebra

Stopgaps: wrongAnswerMarker

Author: Shashwat Singh

Branch/Commit: u/gh-shashwat1002/mod_exponentation_polynom @ c37ccc8

Issue created by migration from https://trac.sagemath.org/ticket/15867

[sagetrac-jakobkroeker]

Stopgaps: wrongAnswerMarker

[jdemeyer]

Description changed:

--- 
+++ 
@@ -1 +1,6 @@
-Some `__pow__` functions just ignore the modulus, see #15777 comment 7.
+
+```
+sage: x = polygen(ZZ)
+sage: pow(x,100,x+1)
+x^100
+```

[shashwat1002]

Branch: u/gh-shashwat1002/mod_exponentation_polynom

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

ff56f10

Implement modular exponentiation in polynomial rings Flint.

[sagetrac-git]

Commit: ff56f10

[slel]

comment:8

Thanks! Please add your full name in the "Authors" field.