Reflection support for unboxed f64 slots

* module/system/vm/assembler.scm (emit-definition): Add representation
  field.
  (write-arities): Emit representations into the arities section.

* module/system/vm/debug.scm (arity-definitions): Read representations.

* module/system/vm/frame.scm (<binding>): Add representation field and
  binding-representation getter.
  (available-bindings): Pass representation to make-binding.
  (frame-binding-set!, frame-binding-ref, frame-call-representation):
  Pass representation to frame-local-ref / frame-local-set!.

* test-suite/tests/rtl.test: Update definition instructions.

* module/language/cps/slot-allocation.scm ($allocation): Add
  representations field.
  (lookup-representation): New public function.
  (allocate-slots): Pass representations to make-$allocation.

* module/language/cps/compile-bytecode.scm (compile-function): Adapt to
  emit-definition change.

* libguile/frames.h:
* libguile/frames.c (scm_frame_local_ref, scm_frame_local_set_x): Take
  representation argument.
  (scm_to_stack_item_representation): New internal helper.

module/language/cps/compile-bytecode.scm

@@ -475,7 +475,8 @@
          (for-each (lambda (name var)
                      (let ((slot (maybe-slot var)))
                        (when slot
-                         (emit-definition asm name slot))))
+                         (let ((repr (lookup-representation var allocation)))
+                           (emit-definition asm name slot repr)))))
                    names vars)
          (when src
            (emit-source asm src))