Remove struct_variant feature gate.

Not ready for merge yet!

While I continue to ponder the ownership guide, I figured I'd do little `rustc` hacking. I currently have this, which I believe removes the feature gate, but, given that the snapshot has the gate turned off, what's the right strategy to fix this? Maybe turn https://github.com/steveklabnik/rust/commit/7903830ffb716ed30adeaa3e39f1c197569b554c#diff-8fe611e80215042c8d89714f4dd6f8baL40 to 'inactive', wait for a snapshot, and then apply this PR afterwards?