refactor: separate backend operations

[aawsome]

This PR refactors indexer and packer into a version which is completely independent from I/O and concurrency but simply provides the functionality. This can then be separated into real "core" functionality.

As a side effect, this PR solves the following topics:

• Ensure that really no duplicate blobs are saved (closes #146)
• Solves potential race conditions (which in theory could lead to data loss) between finalizing packer/indexed and file_saver by using self in the finalize methods.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 61.81102% with 97 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 44.0%. Comparing base (6d3577d) to head (9555d5b).

Files with missing lines	Patch %	Lines
crates/core/src/blob/repopacker.rs	60.1%	53 Missing :warning:
crates/core/src/commands/prune.rs	56.6%	13 Missing :warning:
crates/core/src/blob/pack_sizer.rs	65.0%	7 Missing :warning:
crates/core/src/blob/packer.rs	73.9%	6 Missing :warning:
crates/core/src/commands/repair/index.rs	0.0%	6 Missing :warning:
crates/core/src/index/indexer.rs	89.2%	3 Missing :warning:
crates/core/src/backend/decrypt.rs	33.3%	2 Missing :warning:
crates/core/src/commands/copy.rs	0.0%	2 Missing :warning:
crates/core/src/commands/merge.rs	0.0%	2 Missing :warning:
crates/core/src/commands/repair/snapshots.rs	0.0%	2 Missing :warning:
... and 1 more

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/archiver/file_archiver.rs	63.1% <100.0%> (ø)
crates/core/src/archiver/tree_archiver.rs	69.7% <100.0%> (ø)
crates/core/src/blob.rs	72.7% <ø> (-7.3%)	:arrow_down:
crates/core/src/chunker.rs	50.5% <ø> (-1.2%)	:arrow_down:
crates/core/src/archiver.rs	58.9% <66.6%> (-0.8%)	:arrow_down:
crates/core/src/backend/decrypt.rs	48.1% <33.3%> (+0.2%)	:arrow_up:
crates/core/src/commands/copy.rs	0.0% <0.0%> (ø)
crates/core/src/commands/merge.rs	0.0% <0.0%> (ø)
crates/core/src/commands/repair/snapshots.rs	0.0% <0.0%> (ø)
crates/core/src/index/indexer.rs	91.6% <89.2%> (+45.5%)	:arrow_up:
... and 5 more

... and 19 files with indirect coverage changes

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