field.c: Push field-polynomial property into binary-field base class.

field.c: Push field-polynomial property into binary-field base class.

Using the same code as for prime fields was a mistake, but sharing the
same code between polynomial- and normal-basis fields works just fine;
but there's no point in duplicating the property-table entry when
there's a perfectly good superclass to hang the property off -- and it
already has a property table!