Adapt to mirage/repr#81 (improved binary decoder type)

[craigfe]

See mirage/repr#81.

[gs0510]

The benchmarks fail with this error: :)

+000us  application Will check commit hashes against reference.
+049us [WARNING] index Requested pre-fetch [89778, 117532] is larger than 32768
+049us [WARNING] index Requested pre-fetch [89778, 91390] is larger than 32768
+049us [WARNING] index Requested pre-fetch [89780, 91390] is larger than 32768
+049us [WARNING] index Requested pre-fetch [169022, 268260] is larger than 32768
+049us [WARNING] index Requested pre-fetch [87185, 200449] is larger than 32768
+049us [WARNING] index Requested pre-fetch [87185, 133138] is larger than 32768
+049us [WARNING] index Requested pre-fetch [87185, 200449] is larger than 32768
+049us [WARNING] index Requested pre-fetch [87185, 180003] is larger than 32768
+049us [WARNING] index Requested pre-fetch [229038, 255776] is larger than 32768
tree: internal error, uncaught exception:
      (Failure "CoV1kqf1Vfggz2F9q6vEPTNaWBYUyqmPbvf7HS7ytbtDQDTKgGLx: unknown key")
      Raised at Lwt.Miscellaneous.poll in file "src/core/lwt.ml", line 3072, characters 20-29
      Called from Lwt_main.run.run_loop in file "src/unix/lwt_main.ml", line 31, characters 10-20
      Called from Lwt_main.run in file "src/unix/lwt_main.ml", line 118, characters 8-13
      Re-raised at Lwt_main.run in file "src/unix/lwt_main.ml", line 124, characters 4-13
      Called from Dune__exe__Tree.main in file "bench/irmin-pack/tree.ml", line 382, characters 4-677
      Called from Cmdliner_term.app.(fun) in file "cmdliner_term.ml", line 25, characters 19-24
      Called from Cmdliner.Term.run in file "cmdliner.ml", line 117, characters 32-39
make: *** [Makefile:13: bench-pack-with-trace-replay] Error 125

[craigfe]

Thanks @gs0510. I hope that will be fixed by updating the Index pin to include mirage/index#358, which I've now done.

[craigfe]

Both CI and CB now passing 🎉 (ish – we still have that problem with Odoc)

[craigfe]

Rebased now that the CB is working again. I'm interested to know what the impact of this is now that #1577 has been merged.

[craigfe]

The numbers suggest this gives another 6.6% reduction in minor bytes allocated (and number of minor collections).

I'm in favour of merging mirage/repr#81 and the corresponding PRs in index and irmin.

[craigfe]

I'm planning to merge this today.

[craigfe]

Thanks both for the reviews. Merging now.