lcons structure definition uses :type lcons for its slot

[Anton Kovalenko]

LCONS structure defined in swank.lisp has a type specifier (or .. lcons) for its %CDR slot. From my reading of the standard, it's not guaranteed to work: toplevel defstruct form is required to make its name a valid type specifier at compile-time for /subsequent/ forms, not the defstruct form itself.

Though it seems to work most of the time, it causes a reasonable style-warning in SBCL, and a couple of hard-to-reproduce (load order dependent) build failures as well. It's better to remove type restriction on this slot to be on the safe side.