coercion failures with finite fields with ntl implementation

[videlec]

• GF(2) acting on GF(4)[x]

sage: cm = get_coercion_model()
sage: cm.explain(GF(2), GF(4)['x'])
Action discovered.
...
Univariate Polynomial Ring in x over Finite Field in z2 of size 2^2
sage: cm.explain(GF(2, impl='ntl'), GF(4)['x'])
Traceback (most recent call last):
...
TypeError: unable to coerce <type 'sage.rings.finite_rings.element_ntl_gf2e.FiniteField_ntl_gf2eElement'>

• GF(4) acting on GF(2)[x] (pushout)

sage: cm.explain(GF(4, impl='ntl'), GF(2)['x'])
Coercion on left operand via
...
Result lives in Univariate Polynomial Ring in x over Finite Field in z2 of size 2^2
Univariate Polynomial Ring in x over Finite Field in z2 of size 2^2
sage: cm.explain(GF(4, impl='ntl'), GF(2,impl='ntl')['x'])
Will try _r_action and _l_action
Unknown result parent.

Component: coercion

Issue created by migration from https://trac.sagemath.org/ticket/21786