New CFFI changes mean that bare type references throw warnings

Heya,
This is the first project I've ever tried to contribute to, so sorry in advance if I have missed some common held pull-request etiquette!
Cffi has been updated to allow passing structures as values and they have made some non-backwards compatible changes to how they handle types. As I couldnt get quicklisp to notice my version flags in my asd file I have had a go at updating classimp to use the new style.
One are of note is that with-foreign-slots is now a nightmare as internally it uses foreign-slot-value which will return a struct as a plist. To handle this see the utils:with-foreign-slots* macro which is a quick hack to fix this.
Lastly thankyou for your fantastic work with cl-opengl and classimp, I'm having an amazing time with common lisp and cl-opengl is pretty much the reason for this!
Cheers
Baggers

p.s. These are a first bash at fixing this, I haven’t got any decent tests to run against it.