irmin-pack-tools: Add a printer for index & a readme

[clecat]

This PR add a readme to the irmin-pack-tools folder and a new tool; ppidx:

• The readme contains simple informations on how to use the tools of irmin-pack-tools.
• ppidx aims to give a pretty printer for the informations stored in the index of a store

[codecov-commenter]

Codecov Report

Merging #2214 (90b572c) into main (ce09bb7) will increase coverage by 0.06%. The diff coverage is 68.75%.

:exclamation: Current head 90b572c differs from pull request most recent head e6c25c1. Consider uploading reports for the commit e6c25c1 to get more accurate results

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@@            Coverage Diff             @@
##             main    #2214      +/-   ##
==========================================
+ Coverage   68.01%   68.07%   +0.06%     
==========================================
  Files         135      135              
  Lines       16472    16458      -14     
==========================================
+ Hits        11203    11204       +1     
+ Misses       5269     5254      -15     

Impacted Files	Coverage Δ
src/irmin-pack/unix/dispatcher.ml	75.40% <56.52%> (+12.58%)	:arrow_up:
src/irmin-pack/unix/chunked_suffix.ml	86.08% <100.00%> (+0.12%)	:arrow_up:
src/irmin-pack/unix/lower.ml	62.04% <100.00%> (+0.27%)	:arrow_up:
src/irmin-pack/unix/pack_store.ml	79.32% <100.00%> (ø)
src/irmin-pack/unix/sparse_file.ml	79.16% <100.00%> (+0.17%)	:arrow_up:
src/irmin-pack/unix/traverse_pack_file.ml	68.82% <100.00%> (ø)

... and 1 file with indirect coverage changes

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