haxe.Json refactoring

I was looking into adding pretty-printing to haxe.Json, but it looked like it has to be refactored first, so I did that, specifically:

 * Split `haxe.Json` into `haxe.json.Printer` and `haxe.json.Parser` classes with Haxe own implementation (like it's done in `haxe.xml`)
 * Move target-specific specializations into respective `_std` folders.
 * Inline some trivial functions.

I did run unit tests for every target, adding and removing `-D haxeJSON` (as well `-D old_browser` for JS and `-swf-version 11` for Flash) and it works so far.

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