Copying a `T` inside a compile-time branch

Decided

D-0026, settled by the language’s author: part of spec §18. Older text cites it as 18.26.

A copy only where T is Copy. Written since r13 as a bounded implementation, impl<T: Copy> Copy for Buffer<T>: Buffer<f32> copies, Buffer<File> is move-only, and inside the copy T is Copy because the implementation says so (r12 had the same effect through an implicit rule). Every copyable type satisfies Copy, so fn dup<T: Copy> can say what it needs. Bounding the container itself (Buffer<T: Copy>) was rejected because it would shut out every element type that owns a resource (§8.2, §8.3)