feat: use key-value store for analysis data

[dundee]

To be able to process big amounts of data, we need to store the analysis data on disk instead of memory.

Closes #118.

[codecov]

Codecov Report

Attention: 135 lines in your changes are missing coverage. Please review.

Comparison is base (4e65a50) 93.58% compared to head (059f26b) 90.36%.

Files	Patch %	Lines
pkg/analyze/stored.go	78.73%	47 Missing and 10 partials ⚠️
pkg/analyze/storage.go	79.31%	13 Missing and 5 partials ⚠️
report/export.go	26.08%	17 Missing ⚠️
stdout/stdout.go	57.89%	16 Missing ⚠️
tui/actions.go	0.00%	15 Missing ⚠️
cmd/gdu/app/app.go	27.27%	6 Missing and 2 partials ⚠️
internal/common/ui.go	0.00%	2 Missing ⚠️
pkg/analyze/file.go	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
- Coverage   93.58%   90.36%   -3.22%     
==========================================
  Files          29       31       +2     
  Lines        2806     3250     +444     
==========================================
+ Hits         2626     2937     +311     
- Misses        139      255     +116     
- Partials       41       58      +17     

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