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).
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