let-like syntactic abstraction

A let-like macro for declaring fetches withouth worrying about dependencies:

``` clojure
(flet [posts (latest-posts 10)
post-scores (u/traverse score posts)
authors (u/traverse user posts)]
(map (fn [post score author]
(merge post {:author author :score score}))
posts
post-scores
authors))
```

which will be translated to calls to the combinators under the hood, resolving the data dependencies based on the code.