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

Right, so it seems like it's more correct to state "produces an unboxed that implements the `Fn(int, int) -> int` trait" since this is the most specific statement: every closure implements `FnOnce`, but only a limited subset implement `FnMut` and an even smaller subset implement `Fn`.