Add typed/json which exports almost all of json

This doesn't include `jsexpr?`, nor does it support for custom JavaScript nulls, but is otherwise entirely functional. The former has been excluded due to its inability to be typechecked properly (see [this SO question](http://stackoverflow.com/questions/27304901/how-can-i-use-jsons-jsexpr-predicate-with-typed-racket) and [this mailing list thread](http://lists.racket-lang.org/users/archive/2014-December/065095.html)). The latter has been excluded for similar reasons as well as its relative uselessness.

I've been using these type definitions in working code for some time, and I've encountered no problems despite those missing bindings.