Prohibit complex usage of context

[multimeric]

I enjoy the flexibility of JSON-LD, but I suspect that a lot of the tooling makes assumptions about structure that we haven't actually documented in the spec. Here are some suggestions that might help provide a simpler encoding format:

• Ban @context anywhere except at a top level graph. JSON-LD supports adding @context almost anywhere, but this is confusing and complex to parse
• Ban any keyword redefinition. JSON-LD 1.1 allows you to, for example:
  • Redefine another field to mark @id, such as {"foo": "@id"} (see 4.2.2.14 in the spec)
  • Redefine another field to mark @type, such as {"bar": "@type"} (see 4.2.2.4 in the spec)

There are probably some others I haven't thought of yet, but this is a start.

[ptsefton]

Putting in the v2 milestone for consideration (I agree BTW)