Improve polynomial templates

[jdemeyer]

The files src/sage/rings/polynomial/polynomial_template.pxi and src/sage/libs/flint/nmod_poly_linkage.pxi are a mess:

1. They are almost completely undocumented
2. The functions in it cannot be interrupted (see for example #17470)
3. celement_pow should really be split up in 2 functions: one for the modular case, one for normal powering
4. This shouldn't be needed:

        except OverflowError:
            return Polynomial.__pow__(self, ee, modulus)

Component: basic arithmetic

Issue created by migration from https://trac.sagemath.org/ticket/17476

[jpflori]

comment:2

We should surely have a look at the files organization for p-adics templates as these were written much more recently and possibly in a cleaner way.