Directly implement ToJSON on @ instead of an intermediate ToJSONObject => ToJSON to avoid confusing the compiler

Reverse associativity of '@': we want to enrich metadata on the right of lines, without having to explode existing structures. A paragraph with a class should be just that, and not an article combined to a paragraph number and a class: (Article @ Paragraph) @ Class, not Article @ (Paragraph @ Class)

* Add optparse-applicative parsers for input / output parameters
* Rework metadata types and the way they compose (splitting Article into
  PrimaryKey and Entry)