Add support for ‘uniqueItems’

[Baggz]

This attribute indicates that all items in an array instance MUST be unique (contains no two identical values). Two instance are consider equal if they are both of the same type and: are null; or are booleans/numbers/strings and have the same value; or are arrays, contains the same number of items, and each item in the array is equal to the corresponding item in the other array; or are objects, contains the same property names, and each property in the object is equal to the corresponding property in the other object.