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

> and thus the `FnOnce(int, int) -> int` trait by inheritance

The traits don’t inherit each other at the moment, but [it was discussed as an alternative](https://github.com/nikomatsakis/rfcs/blob/c3cf523156308cd72a739e0b94eca230ecfa7ff5/active/0000-closures.md#unresolved-questions) in the unboxed closures RFC. Should the inheritance between traits be added to this RFC?