Turn TokyoCabinet into a submodule

This should ease updating the version of @Incubaid/tokyocabinet shipped with/used by `camltc`, and adds some more descriptive information to the version info, without the need to keep it up-to-date manually:
```
$ grep dependencies _build/camltc_version.ml
let dependencies = "Incubaid's tokyocabinet fork: 2eed757b82bd2854c729ad5f4c8cc82169f4bc51 3rd-party/tokyocabinet (1.4.48-18-g2eed757)"
```