update for trivial-freeimage

I finished making trivial-freeimage loadable with asdf. I also wrote a macro called with-dib (much thanks to zach for suggesting this).

(asdf:oos 'asdf:load-op :trivial-freeimage)
(use-package :trivial-freeimage)

(with-dib (dib "24.tif")
   (list (get-height dib)
         (get-width dib)))

with-dib will load an image for you and unloads it as well. It uses unwind-protect to ensure the unloading of the image in case of error.