* libguile/vm-engine (box-ref, box-set!): Instead of aborting if a box
isn't a var, call out to vm_error_not_a_variable. This makes these
instructions equivalent to variable-ref/variable-set!.
(vector-set!): Rename from vector-set.
* module/language/cps/compile-rtl.scm (emit-rtl-sequence): Add
variable-set! case, and adapt vector-set!.
* module/language/cps/primitives.scm (*rtl-instruction-aliases*): Add
variable-ref / variable-set! aliases to box-ref / box-set!.