Automatic fields on insert

[kpcyrd]

Right now we deserialize directly into NewOwnedUrl (for example). We should change this to something else which is an alias to NewOwnedUrl or a new type that implements Into<NewOwnedUrl> (or something similar to return a result).

This allows us to have some fields automatically set by other fields. For example we could have a path field that's automatically in sync with value, or we could fill the family field in ipaddrs automatically.