Unpublicize reexports, unprefix JSON type aliases

The type aliases json::JsonString and json::JsonObject were originally
prefixed with 'json' to prevent collisions with (at the time) the enums
json::String and json::Object respectively. Now that enum namespacing
has landed, this 'json' prefix is redundant and can be removed:

json::JsonArray -> json::Array
json::JsonObject -> json::Object

In addition, this commit also unpublicizes all of the re-exports in this
JSON module, as a part of #19253

[breaking-change]
auto merge of #19224 : frewsxcv/rust/unprefix-json-types, r=alexcrichton

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`