Halve() instead of Multiplication by 1/2

[yelhousni]

For all curves, at each step in the Miller loop computation, DoubleStep() is invoked. There are two field multiplications by 1/2 which can be implemented as a Montgomery-like Halve(a) function.

if a is even:
  a = a >> 1
else:
  a = a + modulus // no reduction
  a = a >> 1