New Field Restriction: Enforce uniqueness of elements within array fields

[joneubank]

Summary

Add a new field-level restriction that enforces all elements within an array field are unique (no duplicates within a single record's array value).

This is a design change issue. Introducing a new restriction requires design decisions before implementation can begin.

Motivation

Currently there is no way to declare that array elements must be distinct. For example, a field with isArray: true can silently accept ["A", "A", "B"] with no validation error.

Scope

This issue covers the design and documentation of the new restriction, including:

• How the restriction is declared in a field definition
• How validation is performed when the restriction is present
• How validation errors are reported

Related

Originally raised in #457.

[joneubank]

Before committing to a new restriction definition, its worth considering what other cases there may be for the contents of an array.

I think it is sensible to also add a restriction on the size of arrays (use a range object to define min/max elements).