Create a WikiLambda API that performs validation on a given ZObject.

Need to decide on how validation works. I see two main models:

1. it returns a list of Z5/Error ZObjects. If the list is empty, validation passed. Otherwise, the list contains as many independent errors as possible.
2. return the ZObject itself unchanged if valid, otherwise return an Z5/Error ZObject (we should have an error type that says "multiple errors, which are")