Restore compat Services_JSON (class-json.php), json_encode, json_decode

[nacin]

A fair number of servers and distributions are, by default, missing the JSON extension for PHP. The number is actually a bit surprising, and is enough that we need to go forth with restoring these.

While core does not heavily rely on JSON, a number of actions will fail without it, and a lot of plugins rely on it. Without a compat layer, we'll see fatal errors.

Plugins, coincidentally, have relied on the existence of Services_JSON in class-json.php for unknown reasons, given they should have been using json_encode() and json_decode() directly since 2.9. Restoring this file in place should fix some of those compatibility issues, though I'd like to still get all of those plugins to change their bad ways.

[nacin]

In [18404]:

Restore compat for json_decode and json_encode. fixes #18015 for trunk.

[nacin]

In [18405]:

Restore compat for json_decode and json_encode. fixes #18015 for 3.2.