rollup merge of rust-lang#19224: frewsxcv/unprefix-json-types

Addressing the issues brought up in [this thread](rust-lang#19114 (comment))

This pull request:

* Unpublicizes reexports
* Renames type aliases:
 * `json::JsonArray` ☞ `json::Array`
 * `json::JsonObject` ☞ `json::Object`

Author: alexcrichton