feat: sequential scanning

[dundee]

Sequential scanning is intended to be used on rotating HDDs.

Fixes #316

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (fcc47c5) 90.17% compared to head (7199296) 90.12%.

Files	Patch %	Lines
pkg/analyze/sequential.go	96.52%	3 Missing and 1 partial ⚠️
cmd/gdu/app/app.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
- Coverage   90.17%   90.12%   -0.05%     
==========================================
  Files          31       32       +1     
  Lines        3347     3465     +118     
==========================================
+ Hits         3018     3123     +105     
- Misses        267      276       +9     
- Partials       62       66       +4     

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