Support (:list elt-type) for function parameters

[notmgsk]

When we want to pass in a C-allocated array, we have to use :pointer as the type in sbcl-librarian:define-api and then manually reinterpret that pointer as a lisp list.

It would be convenient if we supported a type such as e.g. (:list :int) which would automatically do the unpacking before handing the list to the associated Lisp function.