RFC: Split the `||` unboxed closure form into two forms and remove the capture clauses and self type specifiers

So, the only distinction between `Copy` and non-`Copy` types that are used by-value inside the closure is the size of the closure itself? (i.e. non-`Copy` types are stored inline in the closure environment but `Copy` types only have a reference.)

Is this a special case that's actually worth having? It seems like "anything used by-value is captured by value" isn't a crazy rule?