Implement signed digit-recoding

[einar-taiko]

Describe the feature you would like

This issue tracks the progress of https://github.com/privacy-scaling-explorations/halo2/issues/187

(mandatory, 1/2 buckets memory requirement) implement signed digit-recoding. wNAF that everyone uses is problematic, it requires precomputation and allocation of all the recoded scalar ahead of times. This is extra memory usage, extra latency, hard to port to GPU. wNAF has 2 draws: being signed recoding so using 2x less space than unsigned, and optimally maximizing the chance of adding zero, i.e. "no cost". But in MSM that second part basically never happens because the number of bits we look at at once grows up to c=16 (or k=16 in Halo2 usual denomiation). Instead Booth encoding can be used for to provide 1, with no precomputation, drastically reducing memory usage.

Additional context

No response