XCAR/XCDR as lvalues

Many of the recent performance changes included replacing Fsetcar() and Fsetcdr() with XCAR/XCDR on the left side of an assignment. After thinking about it, I must conclude that these changes are dangerous in the general case where we don't know if the cons is writable or not. Fsetcar() contains a call to CHECK_IMPURE, while XCAR does not. This means that functions like Fdelq() can well cause a coredump (instead of a mere error) when called on a pure cons, at least on architectures where purespace is read-only. If we plan to deal with this, I think we need an lvalue version of XCAR and XCDR, e.g.: #define XSETCAR(conscell, newcar) do { CHECK_IMPURE (conscell); XCAR (conscell) = newcar; } while (0) /* And ditto for XSETCDR. */ This macro would allow us to deal correctly with read-only conses, while still avoiding the price of a function call to Fsetcar(). (In fact, on *real* Unixes, we could simply use sigaction() to trap SIGSEGV and have the handler check whether the SEGV was caused by modifying a pure cons, and signal an error in that case. Of course, this would not work on Linux.)