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 Insert at a given position in Object maciejhirsz/json-rust#194 to be fixed, if it is someday.

[timothee-haudebourg]

Fixed with json_syntax::Object::insert_front method used here.