Add type alias for function and macro for defining functions with type specifier

`fn' is a convenience macro for defining functions with a type
specifier.  Only really matters for `sbcl` and other hard-optimising
Lisp interpreters which actually take these seriously.

packages.lisp

@@ -21,7 +21,8 @@
   (:use :cl)
   (:export
    :_ :--> :->>
-   :while))
+   :while
+   :-> :fn))
 
 (defpackage main
   (:use :cl :lib.macros)