For @dsheets: <json constr=...> annotation is now <json tag_field=...>

`<json constr=...>` is now `<json tag_field=...>`, whose behavior is easier to guess for the unfamiliar reader of an atd file.

Besides this, the branch provides:
* a bit more context in the description of the `-j-strict-constrs` option
* a simple commented example in the test file; this is just for illustration purposes, there's no test using that type definition. We might want to use it as a base example in the documentation.

Also, I was wondering if you had a good name for that feature (something like "JSON objects as sums").