* 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!.
* libguile/vm-engine.c (prompt): Adapt to explicitly set the saved SP so
we know how many incoming values the handler will receive, and to make
escape-only? a flag.
* module/language/cps/compile-rtl.scm (emit-rtl-sequence): $prompt
should only be found in a "seq" context, as it just pushes on a prompt
and doesn't bind any values. On the other hand it should emit
appropriate code for the handler to bind its values, so do that.
* module/language/cps/slot-allocation.scm ($cont-allocation): Add a note
that proc-slot is used by prompts as well.
(allocate-slots): Compute the allocation of a prompt handler's args.
* module/language/tree-il/compile-cps.scm (convert): Use "unwind"
instead of the nonexistent "pop-prompt".
* module/system/vm/disassembler.scm (code-annotation): Adapt to change
in prompt VM op.
* libguile/vm-engine.c (receive-values): Add an ALLOW-EXTRA? flag in
unused bits of the third word. Without it, receive-values will check
for the exact number of incoming values.
* libguile/vm.c (vm_error_wrong_number_of_values): New error case.
* module/language/cps/compile-rtl.scm (emit-rtl-sequence): Adapt to add
the ALLOW-EXTRA? flag.
