remove the coercion from zmod n to other rings

[jcommelin]

We should remove the coercion, and just use zmod.cast(_hom) everywhere. The coercion causes more trouble than it is worth.
This coercion is currently used in evil ways, by coercing from zmod n to int. Here zmod.val should be used instead.

[eric-wieser]

zmod.val doesn't do the right thing to get

This coercion is currently used in evil ways, by coercing from zmod n to int. Here zmod.val should be used instead.

(zmod.val x : ℤ) sends x = (-37 : zmod 0) to 37, which sounds undesirable.