Support of Set

Is there a plan to support [Set](https://ruby-doc.org/stdlib-2.6.5/libdoc/set/rdoc/Set.html)?

I am trying to use dry-types in place of Virtus in [Grape](https://github.com/ruby-grape/grape/issues/1347). Currently, Set is supported because Virtus supports it.

I am think of the same interface as Array has:

```ruby
PostStatuses = Types::Set.of(Types::Coercible::String)
```

Start by comparing the existing Array type interface with Ruby's Set API, using the linked Set documentation as reference. The proposed Types::Set.of(Types::Coercible::String) should be supported, with tests showing that Set values are accepted and string elements are coerced.

Written by the indexing model from the issue text.