Add an interlace function

Works almost exactly like zip, except it can takes lists of different lengths:

```
interlace [1, 2, 3], ['a', 'b'] # => [1, 'a', 2, 'b', 3]
```

I'm not entirely sure about the order of arguments, it may make more sense to put `shorter` first, although I can't really see the curried form used much.