improve doctest coverage of rings/polynomial/polynomial_compiled.pyx from 0% to 100%

[williamstein]

Part of metaticket #12024.

Just a bunch of little classes for working with "optimized" polynomials, which is actually used somewhere, evidently. Not a single example. Either doctest it completely, by following how it is used by polynomial_element.pyx, or delete it because maybe fast_callable is better? But watch out for large degree, since fast_callable segfaults if the degree is large, so this polynomial_compiled.pyx might have some real value.

Component: algebra

Issue created by migration from https://trac.sagemath.org/ticket/12033

[roed314]

Description changed:

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+++ 
@@ -1 +1,3 @@
 Part of metaticket #12024.
+
+Just a bunch of little classes for working with "optimized" polynomials, which is actually used somewhere, evidently.  Not a single example.  Either doctest it completely, by following how it is used by polynomial_element.pyx, or delete it because maybe fast_callable is better?  But watch out for large degree, since fast_callable segfaults if the degree is large, so this polynomial_compiled.pyx might have some real value.