faster GT membership

[yelhousni]

• BN:
  g^(6*u^2) == g^p
• BLS12:
  g^(p^4+1) == g^(p^2) && g^p == g^u
• BW6:
  g^(p^2+1) == g^p && g^(3p) == g^(13u^6 − 23u^5 − 9u^4 + 35u^3 + 10u + 19)

Using Granger-Scott CyclotomicSquare, and Conjugate for inverse once the first check is verified (g in the cyclotomic subgroup).

[gbotrel]

lgtm, just need to add GT subgroup check when deserializing on GT objects at the curve level.