Add low-level support for unboxed 64-bit unsigned ints

* libguile/frames.c (enum stack_item_representation)
* libguile/frames.c (scm_to_stack_item_representation):
  (scm_frame_local_ref, scm_frame_local_set_x): Support 'u64 slots.
* libguile/frames.h (union scm_vm_stack_element): Add as_u64 member.

* libguile/vm-engine.c (SP_REF_U64, SP_SET_U64): New helpers.
  (scm->u64, u64->scm): New instructions.

* module/language/cps/cse.scm (compute-equivalent-subexpressions):
  Scalar replacement for u64->scm and scm->u64.

* module/language/cps/effects-analysis.scm (scm->u64, u64->scm): Add
  cases.

* module/language/cps/slot-allocation.scm (compute-var-representations):
  (allocate-slots): Represent the result of scm->u64 as a "u64" slot.

* module/language/cps/types.scm (&u64): New type.
  (scm->u64, u64->scm): Add support for these ops.

* module/system/vm/assembler.scm (write-arities):
* module/system/vm/debug.scm (arity-definitions): Support u64
  representations.

libguile/frames.h

@@ -92,6 +92,7 @@ union scm_vm_stack_element
   scm_t_uint32 *as_ip;
   SCM as_scm;
   double as_f64;
+  scm_t_uint64 as_u64;
 
   /* For GC purposes.  */
   void *as_ptr;