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

I thought about that, but I don’t think it’d be a source of bugs at all—it’d be almost entirely invisible to the user. From what I can tell, the non-escaping closures in the RFC are accepted in every place the escaping ones are. The only difference is *how* they do things. I am merely proposing that the non-escaping closures move whenever possible, and that the now-redundant escaping closures be removed.

I suppose the problem with this method is that a lot (most?) of the time you don’t want to move the values into the closure. Perhaps the proposed non-escaping closures could stay as-is, and the escaping ones could be removed completely. The only advantage I see in the escaping closures is that they move things by value more often, which often isn’t even an advantage. Choosing by-ref by default is the most extensible option, as one can still opt in to the old behaviour; e.g., `|| { let foo = x.clone(); drop(x); foo }`. Perhaps I should just rethink the RFC’s proposed escaping closures as sugar for the above for when that functionality is needed.