Master issue for a declarative, reactive, data-driven, or whatever else you want to call it syntax for Go

It's easier to build one of these on top of an existing system than it is to build one of the existing systems on top of one of these, so this would definitely sit on top of package ui, rather than replace it entirely.

That being said, I don't want a One True Standard for this, or at the least, I don't have any particular feeling in favor of one over the other. I'm also not sure if such a thing should be Go-specific or not. The primary goal of this issue is to have one single place for this discussion.