SList improvements

I noticed that concatenation of two `SList`s wasn't working because `SList` itself missed `popFront` required by InputRange. So, simple alias fixed this.

Also someone wrote documentation comment about `opBinaryRight`, but forgot to add actual method. I fixed that.
It looks like, but it's not.`Range` accepts a root node and then exposes it by the same code as `SList` does.

Take a look as `Range` do it: [line 102][0]. And `SList`: [line 184][1] and [line 308][2].
[0]: https://github.com/D-Programming-Language/phobos/blob/deb84d53ec799a57931c7e8da039938a5dbd142a/std/container/slist.d#L102
[1]: https://github.com/D-Programming-Language/phobos/blob/deb84d53ec799a57931c7e8da039938a5dbd142a/std/container/slist.d#L184
[2]: https://github.com/D-Programming-Language/phobos/blob/deb84d53ec799a57931c7e8da039938a5dbd142a/std/container/slist.d#L308