Replace car, cadr and cdr with CL equivalents

`car`, `cadr` and `cdr` are both ancient and cryptic.
Since we're using cl anyways it's a much better idea to
use `first`, `second` and `rest` (not to mention they'll be
more familiar to Clojure hackers).
Merge pull request #297 from bbatsov/cl-style

Replace car, cadr and cdr with CL equivalents