Added asynchronous strings and improved extractor

[Niraj-Kamdar]

I have created FileIO class for asynchronous file IO which just run synchronous IO in asyncio executor as mentioned by @pdxjohnny and splited util module into util and async_utils modules.

[codecov-commenter]

Codecov Report

Merging #746 into master will decrease coverage by 5.41%.
The diff coverage is 83.92%.

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
- Coverage   86.97%   81.55%   -5.42%     
==========================================
  Files         125      126       +1     
  Lines        2372     2445      +73     
  Branches      275      285      +10     
==========================================
- Hits         2063     1994      -69     
- Misses        245      381     +136     
- Partials       64       70       +6     

Flag	Coverage Δ
#longtests	81.55% <83.92%> (-5.42%)	⬇️

Impacted Files	Coverage Δ
cve_bin_tool/util.py	84.00% <ø> (-5.54%)	⬇️
cve_bin_tool/strings.py	73.33% <42.85%> (-26.67%)	⬇️
cve_bin_tool/extractor.py	58.69% <57.14%> (+2.07%)	⬆️
test/utils.py	86.84% <66.66%> (-9.83%)	⬇️
cve_bin_tool/async_utils.py	88.54% <88.54%> (ø)
test/test_extractor.py	97.80% <95.65%> (+23.29%)	⬆️
test/test_strings.py	100.00% <100.00%> (ø)
test/test_json.py	61.76% <0.00%> (-29.42%)	⬇️
cve_bin_tool/cvedb.py	53.14% <0.00%> (-29.25%)	⬇️
cve_bin_tool/checkers/xml2.py	61.36% <0.00%> (-27.28%)	⬇️
... and 7 more

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