Reduce main minimal trace by recording only memory reads

[fractasy]

Today we are recording [a,b] for every step.
If we store only memory reads that are not memory-mapped registers, we would create a vector of read values, in read order, which is deterministic. Since 90% of the memory usage is for registers, this would reduce the length of the trace, and therefore the memory requirements.
This is similar to the zkEVM input db concept, but storing [value] in execution read order instead of the initial set of [key, value].