Better type intellisence for `as_recur_fn` macro by using `$fn_name` as the struct name.#3

Currently, the `as_recur_fn` macro expands to defining a struct `RecurFnImpl` and constructing it as the result. So, the type intellisence of a value by it will be `RecurFnImpl`.  It's not very clear where the value is from. It could be better if the type intellisence is something like `tests::as_recur_fn_works::fact`, which can be achieved by using `$fn_name` instead of `RecurFnImpl` as the struct name.