Per-curve towering

[mratsim]

Currently the tower of extension fields is globally enforced:

constantine/constantine/math/extension_fields/towers.nim

Lines 34 to 61 in 52a8fc4

	type
	NonResidue* = object
	## Non-Residue
	##
	## Placeholder for the appropriate quadratic, cubic or sectic non-residue
	QuadraticExt*[F] = object
	## Quadratic Extension field
	coords*: array[2, F]
	CubicExt*[F] = object
	## Cubic Extension field
	coords*: array[3, F]
	ExtensionField*[F] = QuadraticExt[F] or CubicExt[F]
	Fp2*[Name: static Algebra] =
	QuadraticExt[Fp[Name]]
	Fp4*[Name: static Algebra] =
	QuadraticExt[Fp2[Name]]
	Fp6*[Name: static Algebra] =
	CubicExt[Fp2[Name]]
	Fp12*[Name: static Algebra] =
	CubicExt[Fp4[Name]]
	# QuadraticExt[Fp6[Name]]

However, ideally this config is defined per-curve as for Fp6 BW6 curves are more optimal as Fp -> Fp3 -> Fp6 while for BN and BLS12, the towering is Fp->Fp2->Fp6 (though ultimately Fp->Fp4->Fp12 is better for multiplication).