Exponentiation in Fpk, cyclotomic Fpk and GT

[yelhousni]

• exp in Fpk uses a 2-bit window
• exp in Fpk uses 2-NAF and Granger-Scott cyclotomic squaring
  • TODO: use a window
• exp in GT uses Frobenius GLV
  • TODO: use 2-NAF and a higher-dimensional GLV decomposition (see also 4-dim and 8-dim GLS #78 and 2-NAF in G1/G2 scalar multiplication #132)