init and isIn list helpers

Suggestions for list helpers

```
nix-repl> lib.init [ 1 2 3 4 ]
[ 1 2 3 ]

nix-repl> lib.isIn 2 (lib.init [ 1 2 3 4 ])
true

nix-repl> lib.isIn 4 (lib.init [ 1 2 3 4 ])
false
```