Unpublicize JSON reexports, unprefix JSON type aliases

Addressing the issues brought up in [this thread](https://github.com/rust-lang/rust/pull/19114#discussion_r20614461)

This pull request:

* Unpublicizes reexports
* Renames type aliases:
 * `json::JsonArray` ☞ `json::Array`
 * `json::JsonObject` ☞ `json::Object`
(Don't need the `return` here.)