alg: implement Itoh-Tsujii algorithm

[armfazh]

This algorithm is by far the most used for small addition chains.
It is formulated for binary fields, but the core idea can be adapted to work in prime fields,

https://doi.org/10.1016/0890-5401(88)90024-7

-- didn't check if this algorithm is already covered

[mmcloughlin]

I haven't fully digested this but based on skimming it, I think that method is covered.