Provide --show-interface option to dump interfaces

[alexbiehl]

I have spent lots of time debugging haddock recently and I think this would have helped me a lot. Like ghcs own --show-iface this flag dumps a binary interface file to
stdout in a human (and machine) readable fashion. Currently it uses json as output format.

[alexbiehl]

Currently output is not pretty at all.
$ haddock --show-interface bundled.haddock results in pretty garbled output:

{"link_env": null,"inst_ifaces": [{"module": "$main$BundledPatterns","is_sig": false,"info": null,"doc_map": null,"arg_map": null,"exports": ["$main$BundledPatterns$RTree","$main$BundledPatterns$LR","$main$BundledPatterns$BR","$main$BundledPatterns$Vec","$main$BundledPatterns$Nil","$main$BundledPatterns$:>"],"visible_exports": ["$main$BundledPatterns$Vec","$main$BundledPatterns$Nil","$main$BundledPatterns$:>","$main$BundledPatterns$RTree","$main$BundledPatterns$LR","$main$BundledPatterns$BR"],"options": null,"sub_map": {"$main$BundledPatterns$tsplit": [],"$main$BundledPatterns$textract": [],"$main$BundledPatterns$tail": [],"$main$BundledPatterns$head": [],"$main$BundledPatterns$:>": [],"$main$BundledPatterns$LR": [],"$main$BundledPatterns$BR": [],"$main$BundledPatterns$Vec": ["$main$BundledPatterns$Nil","$main$BundledPatterns$Cons"],"$main$BundledPatterns$RTree": ["$main$BundledPatterns$LR_","$main$BundledPatterns$BR_"]},"bundled_patsyns": {"$main$BundledPatterns$Vec": ["$main$BundledPatterns$:>"],"$main$BundledPatterns$RTree": ["$main$BundledPatterns$LR","$main$BundledPatterns$BR"]},"fix_map": {"$main$BundledPatterns$Cons": {"prec": 5,"direction": "infixr"},"$main$BundledPatterns$:>": {"prec": 5,"direction": "infixr"}}},{"module": "$main$BundledPatterns2","is_sig": false,"info": null,"doc_map": null,"arg_map": null,"exports": ["$main$BundledPatterns$RTree","$main$BundledPatterns$LR","$main$BundledPatterns$BR","$main$BundledPatterns$Vec","$main$BundledPatterns$Nil","$main$BundledPatterns$:>","$main$BundledPatterns2$Empty"],"visible_exports": ["$main$BundledPatterns2$Empty"],"options": null,"sub_map": {"$main$BundledPatterns2$Empty": []},"bundled_patsyns": {},"fix_map": {}}]}```

[hvr]

This will prove very relevant to my hidden agendainterests... thanks for tackling this! :-)

[alexbiehl]

Thank you @dmalikov! Nice work!