Make it possible to use the custom validation type when validating structs

[bobsilverberg]

Currently you cannot use the custom validation type when validating a struct as the custom type expects a method to exist in the object that is being validated. With a struct there is no object.

When a struct is validated it does get wrapped in an object before validation, so if we can find a simple/easy way for a developer to inject a method into that wrapper struct then that would work. I'm not sure what the best approach to that would be, and would welcome any feedback on it.