irmin-pack: introduce Sparse_file abstraction

[art-w]

This PR tries to abstract the mapping/data files coupling behind a Sparse_file module (that we could use to implement the lower volumes?). It's a pretty aggressive refactoring so I would understand if it is not welcomed! It notably gets rid of the dispatcher's accessors because I find it easier to hide the offset computations inside the Sparse/Chunked files blackboxes.

If there's no strong objection, my plan is to add some append-only functions to this Sparse_file that the GC will be able to use to transfer data to the lower. There's also still some refactoring to do in File_manager as it could delegate the opening/closing/etc of the mapping/data file to the new module.

[codecov-commenter]

Codecov Report

Merging #2172 (dd6f3f0) into main (ff45092) will decrease coverage by 0.09%. The diff coverage is 76.15%.

:exclamation: Current head dd6f3f0 differs from pull request most recent head a5ad27c. Consider uploading reports for the commit a5ad27c to get more accurate results

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@@            Coverage Diff             @@
##             main    #2172      +/-   ##
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- Coverage   68.07%   67.98%   -0.09%     
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  Files         134      134              
  Lines       16161    16107      -54     
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- Hits        11001    10950      -51     
+ Misses       5160     5157       -3     

Impacted Files	Coverage Δ
src/irmin-pack/inode.ml	78.96% <0.00%> (ø)
src/irmin-pack/unix/gc_worker.ml	4.41% <0.00%> (+0.06%)	:arrow_up:
src/irmin-pack/unix/import.ml	87.50% <ø> (+5.14%)	:arrow_up:
src/irmin-pack/unix/store.ml	64.11% <ø> (ø)
src/irmin-pack/version.ml	44.44% <ø> (ø)
src/irmin-pack/unix/async.ml	56.60% <40.00%> (+2.75%)	:arrow_up:
src/irmin-pack/unix/dispatcher.ml	58.73% <52.50%> (-17.30%)	:arrow_down:
src/irmin-pack/unix/chunked_suffix.ml	85.58% <60.00%> (-0.27%)	:arrow_down:
src/irmin-pack/unix/sparse_file.ml	82.92% <82.92%> (ø)
src/irmin-pack/unix/pack_store.ml	82.72% <83.33%> (ø)
... and 15 more

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[art-w]

Thanks for the review! I've made some changes (to be squashed after approval) and simplified a bit the file manager (... I'm trying to expose less and less of the Mapping_file, although the GC worker still need this internal access until the sparse file provides "append" operations)