Parallelize file parsing

[YuriRomanowski]

Clarification and motivation

This topic is a part of #221.
After we read file contents, we should perform parsing of the files, which is (in theory) pure action and can be parallelized.
But we use C library under the hood, so the parallelization may be tricky. Here we can try some approaches and discuss results.

Acceptance criteria

• Some approaches of parallelization are tried
• We decided how to handle foreign calls during parsing for parallelization
• Speedup is obtained and proved with measurements

[YuriRomanowski]

I uploaded some commits where different variations of xrefcheck can be load-tested (in branch YuriRomanowski/#247-parallelize-file-parsing-scaffolding):

• Original version (from master) with lazy readFile: 2a959d0
• Replace lazy readFile with strict one: b3368c3
• Force reading files and then process them in parallel using Eval monad: a1d5f56
• Force reading files and then process them using mapConcurrently: 8f65374
  The latter two ones produce similar results.

[Martoon-00]

Thanks for this investigation!

I tried, and from what I can see:

• Repo scanning time is not extremely different in all of these scenarios (0.9s / 0.7s / 0.5s / 0.5s)
• My impression was that in the given load testing there was simply no space for parallelization (this is what we saw on this picture. Sparks tab shows that a few sparks were bind to different cores, but most of them went to one core, probably simply because parsing was fast enough to process it all.
• I tried to create 4 dummy markdown files, 50Kb each, and sparks solution showed 4 cores being used.

(the selected area corresponds to repo scanning time)

Although I'm not exactly sure why "Activity" graph at the top shows so few CPU feed.