Do not rely on subfield to create the base ring in EquiangularPolygons

[videlec]

In order to compute the slopes, instead of going through the minimal polynomial of cosines and sines and call projectivization(cos, sin) we could directly compute the minimal polynomial of the tangent.

Note that the only cases where E._base_ring differs from E._cosines_ring is when N % 4 == 0. In that case [E._cosines_ring:E._base_ring] = 2.

[videlec]

Though the minimal polynomial of tan(pi / N) is not nice...