Fix union Lisp_Fwd * alignment bug

Fix union Lisp_Fwd * alignment bug

It's not portable to cast (e.g.) struct Lisp_Objfwd * to union
Lisp_Fwd * and then back again, because the compiler can then assume
that the pointer is aligned for union Lisp_Fwd * when accessing
the struct Lisp_Objfwd * components, and this assumption might
be incorrect becase we don't force that alignment.
* src/lisp.h (lispfwd): New type, replacing ...
(union Lisp_Fwd): ... this type, which was removed.
All uses changed.
(SET_SYMBOL_FWD): 2nd arg is now void *, not lispfwd.
All uses changed (casts no longer needed; they were
not portable anyway).