[Kotlin] Add kotlin.collections.Set support for unique item arrays

[mtraynham]

Adds support for kotlin.collections.Set array types, when interpreting ArraySchema.getUniqueItems()

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@jimschubert @dr4ke616 @karismann @Zomzog

[wing328]

@mtraynham thanks for the PR, which looks good to me.

[mtraynham]

Thanks @wing328 !

[jimschubert]

Doesn't this cause compilation error because instantiation currently only knows of maps and lists?

[jimschubert]

Oh. I see that the change affects the generic type construction, and we're not changing actual type mapping for the array type (which would need a setOf instantiation type).

[wing328]

@mtraynham thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569