Support multiple IRIs for `@reverse`

Hi all,

It would be great to allow multiple property IRIs in the @reverse property. This would allow users to transform JSON-LD to other formats like N-Quads more easily without having to manipulate the output format to add additional triples with the extra properties they want to add.

For example, I have some JSON-LD like this:

{
  "@id": "ex:BEA/code/FREQ",
  "@type": "skos:ConceptScheme",
  "skos:notation": "FREQ",
  "entries": [
    {
      "@id": "ex:BEA/code/FREQ/A",
      "@type": "skos:Concept",
      "skos:prefLabel": "Annually",
      "skos:notation": "A"
    },
    {
      "@id": "ex:BEA/code/FREQ/Q",
      "@type": "skos:Concept",
      "skos:prefLabel": "Quarterly",
      "skos:notation": "Q"
    }
  ]
}

I'd like to have a context entry like this:

    "entries": {"@reverse": ["skos:hasTopConcept", "skos:inScheme"]}

This use-case is based on Example 14 from the RDF Data Cube Specification.

I know that the feature set is frozen for 1.1, I just wanted to put this out there for consideration for the next version.

I guess the trickiest part would be handling the expanded form, which looks like this currently:

{
  "@reverse": {
    "http://www.w3.org/2004/02/skos/core#hasTopConcept": [
        {
          "@id": "ex:BEA/code/FREQ/A",
          "@type": "skos:Concept",
          "skos:prefLabel": "Annually",
          "skos:notation": "A"
        },
    ]
  }
}

We can't have keys that are arrays, so we'd need to restructure it. I'm not sure if it's possible to do that in a backwards-compatible way, which might be a deal-breaker.

Let me know what you think. Thanks!

Begin with issue #372 and the JSON-LD 1.1 specification sections covering @reverse, expanded form, and processing; compare the proposal with RDF Data Cube Example 14. Done means agreeing on a backwards-compatible representation and defining the required processing behavior; the issue names no files or tests.