Dereference a pointer offset bytes from ptr to an object for reading (or writing when used with
setf) of built-in type type.
;; An impractical example, since time returns the time as well,
;; but it demonstrates %MEM-REF. Better (simple) examples wanted!
(with-foreign-pointer (p (foreign-type-size :time))
(foreign-funcall "time" :pointer p :time)
(%mem-ref p :time))