Add support for uniqueness in collection types

Uniqueness in a collection of items is useful in many contexts, and could present important informations that can be used in documenting APIs and validating request and response values
- Bitfield-like unordered collections of enumerated values
- Sets of scalar types
- Keyed Types

This is supported in a limited fashio using the `@uniqueItems` decorator for Azure APIs, but the functionality could be useful in many API contexts.

This could be resolved using value assertions over existing collection types, or by using types like `Set`, `OrderedSet` that inherently require unique values.