Rename JSONDecodable to Decodable

[gfontenot]

Decodable objects are able to be decoded, returning a
Decoded<DecodedType>. This is a nice synergy, and leaves us open to
other non-JSON operations in the future.

[tonyd256]

I like the change in general because I think it would be nice to get away from JSON references in general. This is getting us one step closer. Using Value instead of JSON would be the final nail in the coffin.

I'm a 👍 for this!

[gfontenot]

I think right now, JSON is at least conceptually tied to parsing JSON. If an example comes up where we (or someone else) has, for example, a YAML parser that works with the JSON type without modification, I think it would make sense to change it at that point. And we could always typealias JSON = Value if it comes to that to maintain backwards compatibility.

[gfontenot]

ping @thoughtbot/ios and @hyperspacemark for feedback please

[dazmuda]

👍 I really like this change. You decode Decodable objects. Even when parsing JSON, the nomenclature decodeing JSONDecodable objects is a bit awkward.

[hyperspacemark]

I dig it. 👍

[jnutting]

I am all in favor of this