irmin-pack: GC adds data to lower

[Firobe]

Progress:

• [x] Basic file management
• [x] Empty volume management
• [x] Volume mapping append
• [x] Volume data append
• [x] Temporary control file creation
• [x] Control file swap
• [x] Proper tests (partially done, reusing the normal GC tests)
• [x] Proper error management
• [x] Documentation, update examples
• [x] Squash everything

[codecov-commenter]

Codecov Report

Merging #2202 (7aafca9) into main (2b975d0) will decrease coverage by 0.08%. The diff coverage is 28.94%.

:exclamation: Current head 7aafca9 differs from pull request most recent head 66328ea. Consider uploading reports for the commit 66328ea to get more accurate results

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@@            Coverage Diff             @@
##             main    #2202      +/-   ##
==========================================
- Coverage   68.02%   67.94%   -0.08%     
==========================================
  Files         135      135              
  Lines       16379    16435      +56     
==========================================
+ Hits        11142    11167      +25     
- Misses       5237     5268      +31     

Impacted Files	Coverage Δ
src/irmin-pack/unix/errors.ml	28.88% <ø> (ø)
src/irmin-pack/unix/gc.ml	77.66% <ø> (-1.96%)	:arrow_down:
src/irmin-pack/unix/gc_worker.ml	4.25% <0.00%> (-0.30%)	:arrow_down:
src/irmin-pack/unix/io_errors.ml	58.33% <ø> (ø)
src/irmin-pack/unix/sparse_file.ml	76.14% <0.00%> (-0.71%)	:arrow_down:
src/irmin-pack/unix/store.ml	64.82% <ø> (+0.11%)	:arrow_up:
src/irmin-pack/unix/lower.ml	58.20% <7.14%> (-11.80%)	:arrow_down:
src/irmin-pack/unix/file_manager.ml	85.27% <80.00%> (+0.83%)	:arrow_up:
src/irmin-pack/layout.ml	83.92% <100.00%> (+0.29%)	:arrow_up:
src/irmin-pack/pack_value.ml	84.61% <100.00%> (ø)
... and 7 more

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[Firobe]

I'm still writing more tests, but I think the core of the PR is ready for (more :p) scrutiny.

[Firobe]

Merging to help testing of migration.