Extend w/fn, let to support destructuring

[holyjak]

Now I must do

(w/fn [latLon]
               [(w/Part latLon 1)
                (w/Part latLon 2)])

In Wolfram, this would be {#[1],#[2]}&.

It would be lovely to be able to write

(w/fn [[lat lon]] [lat lon])

At least for vectors. Maps/Associations might be nice too, not sure how often they are used.