Exporting and importing a JSON Corpus does not roundtrip

While investigating #517 I have noticed that if we export a full corpus as a JSON file, it gets exported using the ToJSON instance of a Corpus data structure, which outputs a JSON object like the one I'm attaching.GarganText_corpus-nodeId-304.json

However, when we import a corpus, we parse the input using the FromJSON instance of the JSONStruct type, which is slightly different, leading to this error:

We should really pick a consistent instance, and have a test in the codebase which test the roundtrip between the import and an export.