Port difffolded.pl as inferno-diff-folded

[jasonrhansen]

Fixes #62

I also changed flamegraph to always sort input lines since the output of diff-folded isn't sorted. This matches the behavior of upstream flamegraph.

[codecov-io]

Codecov Report

Merging #96 into master will increase coverage by 0.54%.
The diff coverage is 98.7%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   88.48%   89.02%   +0.54%     
==========================================
  Files          11       12       +1     
  Lines        1294     1367      +73     
==========================================
+ Hits         1145     1217      +72     
- Misses        149      150       +1

Impacted Files	Coverage Δ
src/flamegraph/mod.rs	95.86% <100%> (+0.07%)	⬆️
src/flamegraph/merge.rs	94.94% <100%> (+0.32%)	⬆️
src/differential/mod.rs	98.38% <98.38%> (ø)

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[jasonrhansen]

I'm bothered enough about the sorting this PR adds to flamegraph that I think we should provide a way to opt out if you know the input is already sorted. We could make compatibility with upstream the default, but have a flag like --no-sort which is basically a promise that the input is already sorted. What do think?

[jonhoo]

@jasonrhansen I think something like --no-sort is a great idea. We may then want to warn if we detect unsorted input though. That can be an entirely local check. Just make sure that each line lexiographically follows the previous one. Maybe even do it really stupidly by only checking the first character.

[jasonrhansen]

I like the idea of detecting and warning about unsorted input when using --no-sort.