Allow custom serialisation of prop values

Prop values of type Set are serialised as `foo={[object Set]}`. It would be much more convenient to serialised them in a similar way to the Arrays (i.e. `foo={Set ['one', 'two', 'three']}`).
This could be coded as an extension if custom serialisation of props were supported or as a specific prop serialiser for Set values.

Thanks,
Alberto

The issue names no files or tests. Start by locating the serializer that handles Array prop values and inspect how unsupported Set values are formatted; done means Set values no longer render as [object Set] and either receive the requested JSX-like representation or can use the proposed custom serialization extension.

Written by the indexing model from the issue text.