Polymorphic array mapping (are you interested in a PR)

[scornflake]

Hi. Interested to know if this is something you'd like to add.

i.e: the ability to write an array of various types (subclasses), and read it back in. All using the same <- operator.

I've written an implementation that works for my own needs, and would be happy to make it a bit more 'tidy' and provide as a PR if there's any interest.

It's only downside is that the reconstruction insn't 100% automatic. It requires a specific closure to instantiate the correct objects upon hydration. Perhaps someone better @ swift that I can make that more general.