add multiple values support to the vm

* libguile/vm-engine.c (vm_run): The bootstrap program now uses mv_call,
  so as to allow multiple values out of the VM. (It did before, because
  multiple values were represented internally as single scm_values
  objects, but now that values go on the stack, we need to note the boot
  frame as accepting multiple values.)
  (vm_error_no_values): New error, happens if you pass no values into a
  single-value continuation. Passing more than one is OK though, it just
  takes the first one.

* libguile/vm-i-system.c (halt): Assume that someone has pushed the
  number of values onto the stack, and package up that number of values
  as a scm_values() object, for communication with the interpreter.
  (mv-call): New instruction, calls a procedure with a multiple-value
  continuation, even handling calls out to the interpreter.
  (return/values): New instruction, returns multiple values to the
  continuation. If the continuation is single-valued, takes the first
  value or errors if there are no values. Otherwise it returns to the
  multiple-value return address, pushing the number of values on top of
  the values.

* module/system/il/compile.scm (codegen): Compile <ghil-values> forms.

* module/system/il/ghil.scm (<ghil-values>) Add new GHIL data structure
  and associated procedures.

* module/language/scheme/translate.scm (custom-transformer-table):
  Compile (values .. ) forms into <ghil-values>.

module/language/scheme/translate.scm

@@ -300,7 +300,11 @@
 
     ;; FIXME: make this actually do something
     (start-stack
-     ((,tag ,expr) (retrans expr)))))
+     ((,tag ,expr) (retrans expr)))
+
+    (values
+     ((,x) (retrans x))
+     (,args (make-ghil-values e l (map retrans args))))))
 
 (define (trans-quasiquote e l x level)
   (cond ((not (pair? x)) x)