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[bot]]

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 :warning:
pkg/analyze/storage.go	79.31%	13 Missing and 5 partials :warning:
report/export.go	26.08%	17 Missing :warning:
stdout/stdout.go	57.89%	16 Missing :warning:
tui/actions.go	0.00%	15 Missing :warning:
cmd/gdu/app/app.go	27.27%	6 Missing and 2 partials :warning:
internal/common/ui.go	0.00%	2 Missing :warning:
pkg/analyze/file.go	83.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ 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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