Improve performance on modulo powers operations

[LukePearson1]

Currently, the modulo exponentiation (or power modulo) in the library is documented under src/traits.rs, and is coded as:
` pub trait Pow {
type Output;

    #[must_use]
    /// Returns  `a^b (mod l)`. 
    fn pow(self, exp: T) -> Self::Output;`.

Exponentiation operations calculates the remainder when the integer a, is raised to the b'th power, and b is divided by the positive integer l. These operations are expensive for a CPU as when they become iterative each multiplication takes at least twice as long as the previous one. As they are not done to an order of (O(log b)).
As these operations are done continuously when representing Edwards points as Ristretto points then the optimisation of the process needs to be worked on. Examples and ideas will be marked below, then implemented and then benchmarked.

[CPerezz]

Since this passes by having performant and ctime addition-chains for the curve, here some resources i found which may be useful.

https://github.com/kwantam/addchain
https://github.com/Eirik0/add-chain-converter