Place `@context` entry on top.

[timothee-haudebourg]

Because of a limitation of the json crate, the compaction algorithm can only attach the @context entry at the bottom of the generated json object. I see two solutions for this:

• Add a compact_in function that would compact a document inside a prepared json object, with the context already attached on top.
• Wait for maciejhirsz/json-rust#194 to be fixed, if it is someday.