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.

dash.el

@@ -27,6 +27,13 @@
 
 ;;; Code:
 
+(defalias '-first-item 'car
+  "Returns the first item of LIST, or nil on an empty list.")
+
+(defun -last-item (list)
+  "Returns the first item of LIST, or nil on an empty list."
+  (car (last list)))
+
 (defmacro !cons (car cdr)
   "Destructive: Sets CDR to the cons of CAR and CDR."
   `(setq ,cdr (cons ,car ,cdr)))

dev/examples.el

@@ -9,6 +9,14 @@
 (defun square (num) (* num num))
 (defun three-letters () '("A" "B" "C"))
 
+(defexamples -first-item
+  (-first-item '(1 2 3)) => 1
+  (-first-item nil => nil))
+
+(defexamples -last-item
+  (-last-item '(1 2 3)) => 3
+  (-last-item nil => nil))
+
 (defexamples -map
   (-map (lambda (num) (* num num)) '(1 2 3 4)) => '(1 4 9 16)
   (-map 'square '(1 2 3 4)) => '(1 4 9 16)