Adding -first-item and -last-item. [magnars/dash.el#17]

`-first-item` is a trivial alias to `car`, so I used `defalias` so
there isn't the additional overhead of a function call. This has
broken `create_docs.sh`.

It seems that `defexamples` expects `symbol-function` to return an
elisp function definition, but `car` is implemented in C.