Add a way to more easily generate a JSON-LD context from a set of keywords+prefixes

[theferrit32]

We can look them up in property-names.edn and namespaces.edn. Should reduce duplicate definition of what terms are and how they should be expanded/compacted.

A function should be able to produce a keywordized jsonld-esque map structure (using genegraph's class/property keywords), and the caller should be able to, without knowing the details about what the map contains:

• serialize to plain JSON
• generate a JSON-LD context to attach to the plain JSON
• expand and compact the JSON using this context