Rewrote --> to take a placeholder symbol as first argument

Instead of exporting cantedraw.lib.macro._ and making anyone who wants
to use the --> macro _require_ importing that specific symbol, let's
just make it so the user has to supply a placeholder name before they
do anything.  This means each package provides its own placeholder
symbol which lowers coupling.

packages.lisp

@@ -20,7 +20,7 @@
 (defpackage cantedraw.lib.macros
   (:use :cl)
   (:export
-   :_ :--> :->>
+   :--> :->>
    :-> :fn
    :while :alist-val
    :$))