Jsonvl

------

* Union types (including nullable)
* Type definitions and references to definitions
* Exception type differentiation between data and schema errors

------

* Improved error messages standardized across JSON types

------

* Type checking for JSON data types (number, string, boolean, object, array, null)
* Number constraints (lt, gt, eq)
* String constraints (in, eq, format)
* Array constraints (max_size, min_size, unique)