Implement a field.Set?

I was curious why Marshmallow didn't have a `field.Set` for Python sets.

I would imagine the code would be mirror identical to `List` (https://github.com/marshmallow-code/marshmallow/blob/dev/src/marshmallow/fields.py#L656) only with a `list` being serialized into a `set`, and deserialized back into a `list`.

Would this be easy to implement? Any pitfalls I should be made aware of?

Start in src/marshmallow/fields.py around the existing List field referenced in the issue and compare its serialization and deserialization behavior. Clarify the intended set/list conversion, then add the Set field and verify that its behavior matches the agreed semantics.

Written by the indexing model from the issue text.