Add missing function /=

Again from the fine guide

* https://gigamonkeys.com/book/numbers-characters-and-strings.html

> The /= function, conversely, returns true only if all its arguments are different values.

```
(/= 1 1)        ==> NIL
(/= 1 2)        ==> T
(/= 1 2 3)      ==> T
(/= 1 2 3 1)    ==> NIL
(/= 1 2 3 1.0)  ==> NIL
```
