Optimize and refactor the various `showdiff` commands.

[JiriPavela]

With the addition of the new folded report variant, there are now multiple implementations of the data processing pipeline in showdiff flamegraph, showdiff report native and showdiff report folded. We should optimize the folded report even more (see #344 as one optimization example) and then refactor and unify the computations in different showdiff commands such that they all use the optimized code.