fix: sort blobs before repacking

[aawsome]

closes rustic-rs/rustic#1680:

• sort blobs-to-repack in prune
• assert that no overlaps occur when coalescing BlobLocations.
• added unit tests for the coalescing.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.1%. Comparing base (f5108f3) to head (7e16a70).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/blob.rs	0.0%	1 Missing ⚠️
crates/core/src/commands/prune.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/blob.rs	63.8% <0.0%> (+5.5%)	⬆️
crates/core/src/commands/prune.rs	63.9% <0.0%> (-0.5%)	⬇️

... and 31 files with indirect coverage changes

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