irmin-pack: Add sequential accessors & clean dispatcher

[clecat]

This PR adds:

• 2 accessors that uses physical offsets instead of the virtual offsets
• A generator for sequences of accessors that iterates trough all physical offsets of a pack file.
  It also removes:
• The root filed of Dispatcher.t, as it is never used.

[codecov-commenter]

Codecov Report

Merging #2090 (f66d223) into main (368f671) will decrease coverage by 0.14%.
The diff coverage is 7.14%.

@@            Coverage Diff             @@
##             main    #2090      +/-   ##
==========================================
- Coverage   64.75%   64.60%   -0.15%     
==========================================
  Files         132      132              
  Lines       15658    15696      +38     
==========================================
+ Hits        10139    10141       +2     
- Misses       5519     5555      +36     

Impacted Files	Coverage Δ
src/irmin-pack/unix/gc.ml	40.54% <0.00%> (ø)
src/irmin-pack/unix/dispatcher.ml	51.18% <2.56%> (-21.86%)	⬇️
src/irmin-pack/unix/ext.ml	61.94% <100.00%> (ø)
src/irmin-pack/unix/snapshot.ml	77.34% <100.00%> (ø)
src/irmin-fs/irmin_fs.ml	81.21% <0.00%> (-1.22%)	⬇️
src/irmin-fs/unix/irmin_fs_unix.ml	68.38% <0.00%> (+0.64%)	⬆️
src/irmin/watch.ml	80.98% <0.00%> (+2.11%)	⬆️

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[Ngoguey42]

Thanks. Ship it