3.0 - Start ORM-less forms

This starts the changes discussed in #5390. I've not started the FormHelper context but I will shortly. I figured this was a first step and separate from the FormHelper integration.

I've intentionally not addressed embedding validators, and schema instances in each other. I'd rather get an end to end form workflow complete for simple forms, and then layer in complex forms. I also don't have a solid grasp on how embedded validators/schema/forms should work yet.
Missing `@param` for `$data`.