Add -snoc

This is like cons but on the end of the list. Useful. And look at this:

```lisp
(defmacro ->> (init &rest forms)
  (-reduce-r '-snoc (reverse (cons init forms))))
```

isn't it pretty? :package: 

(by the way, if you've never seen it, `snoc` (= cons backwards) is a common CS jargon for lists that cons from backward, like `(((nil . 1) . 2) . 3)`, so that's where it comes from)