fix(lint): resolve E410, E415, E605 in irmin; tidy irmin bin/lib
- E605: Split monolithic test_irmin.ml into 10 per-module test files
(test_hash, test_tree, test_backend, test_store, test_codec, test_link,
test_proof, test_commit, test_git_interop, test_subtree)
- E410: Fix doc arg count for any_algorithm, any_to_bytes, any_to_hex
- E415: Add Proof.pp pretty-printer
- Tidy irmin bin commands (cmd_checkout, cmd_import, cmd_info)
- Tidy irmin lib (backend, codec, link) and add irmin.mli
- Fix merlint E331 rule, dune fmt across space-dtn