Improve performance of MSMPrecompWindowSigned::mul

[psalz]

This improves the performance of MSMPrecompWindowSigned::mul in cases where many of the scalars are zero. This is achieved by switching the loop order, adding an early exit condition and avoiding unnecessary allocations. Committing to only a few or even a single non-zero value is frequently needed when doing incremental updates on commitments, for example when updating a single value of a Verkle trie leaf.

I added a benchmark to exercise this, for which on my machine I see a 28% improvement:

msm precomp 16 single idx: 3
                        time:   [1.3312 µs 1.3536 µs 1.3787 µs]
                        change: [-29.653% -28.399% -27.165%] (p = 0.00 < 0.05)
                        Performance has improved.

The performance of committing to all non-zero values remains unaffected.