a48ca3290d
Correct treatment of irrefutable patterns. The old code was wrong in many, many ways. `ref` bindings didn't work, it sometimes copied when it should have moved, the borrow checker didn't even look at such patterns at all, we weren't consistent about preventing values with destructors from being pulled apart, etc. Fixes #3224. Fixes #3225. Fixes #3255. Fixes #6225. Fixes #6386. r? @catamorphism