Document iter().copied() vs iter() for small data types

[Dan-wanna-M]

As the issues rust-lang/rust#106539 and rust-lang/rust#113789 indicate, iter().copied() is more efficient than iter() for small data types because the former one results in less indirections and hence opens up the room for more optimizations. How small is small needs more investigations, but anything <= usize should definitely be small(in fact, the original reply in the issue suggests 8*usize is probably still small enough).

[Shnatsel]

Indeed, there is a comment form a compiler developer that this is extremely difficult for LLVM to optimize, and that putting .copied() early should be encouraged: rust-lang/rust#106539 (comment)