Make decoding lazy

Now we have parse the whole JSON value into the JSON type, but then you have a lot of overhead when you have a lot of properties that you don't use. I tried this with a type DecodableValue that has a helper method to cast the data of the DecodableValue to the type that you want. DecodableValue replaces JSON and also saves a lot of looping and switches.

[gfontenot]

This is something we're definitely looking to do, especially since we're starting to use Argo with more complex JSON structures, like those adhering to JSON:API.