[WIP] Update Dry-Struct/Dry-Types.

[tpendragon]

This should add all the necessary back-ports to prevent having to
release a major version of Valkyrie, but still support the latest
dry-types and dry-struct.

Fixes #434

[tpendragon]

I just ran it against Figgy and it all passes, but there are a couple gotchas that I sooort of feel like I either have to find a way around or release 2.0. Curious about opinions:

1. Instance variables are not set anymore. So code depending on, IE, @title, doesn't work. Would have to get changed to attributes[:title]
2. attributes doesn't change when you define the getters for methods anymore (part of why @title doesn't work). It's just an internally managed hash - so if you call to_h and have overridden the title method, the title in to_h will be the non-overridden version.

[tpendragon]

Part of me feels like both of these issues are undocumented and happenstance quirks of the implementation of the features of the system, and overriding title actually goes against the goals of the project.

The other part feels like this was something a real project has done and I should respect that with a patch to maintain that implementation detail.