Implement CT evenness checking using subtle's Choice.

[CPerezz]

Since is_even() function calls inside the primitive type u64 is_even() function, it will be faster to take a workaround to enable even faster even numbers checking.ç

The workaround will be something similar to:

Let a be a Scalar or FieldElement which we don't know if it's even or not.

It'll be something similar to:

// Check even by applying an AND logical operator, to see if the last bit of `a` is set
// to `1` or `0`. 
pub fn is_even(&self) -> Choice {
   Choice::from((a[0] & 1u64) as u8)
}

Then, with the result, return a Choice which is a subtle type implemented by Isis Agora Lovecruft and Henry de Valence.