halo2-ecc: `ec_double` helper doesn't work for secp256r1

[DreamWuGit]

halo2-ecc/src/ecc/mod.rs helper ec_double helper only work for secp256k1 , not work for p256, need to be updated as following:
if current point is P (x, y)
then
lambda = (3* x ^2 + a) / 2y (mod p)
x_3 = lambda^2 - 2x (mod p)
y_3 = lambda(x_3 -x) (mod p)
the point(x_3, y_3) is the addition result.

[DreamWuGit]

better to make modification compatible to both two curves instead of creating new helper special use for r1 curve.