Short enum variant syntax in some cases

Various permutations of this (enum::Foo, also allowing _ { x: 5 } for structs, etc) come up regularly. Here's a nice summary comment, though it was concentrated more on use in patterns that in calls.

I'd definitely like something here, ideally both for enums and structs. I think the stumbling block is mostly finding a syntax that makes people happy, and potentially naming convention impacts -- _::Yes is clearly poor, but a reasonable enum to make today if it's usually passed as UseCookies::Yes.