Untag values and indexes for all bytevector instructions

* libguile/vm-engine.c (bv-s8-ref, bv-s16-ref, bv-s32-ref, bv-s64-ref):
  Unbox index and return unboxed S32 value.
  (bv-s8-set!, bv-s16-set!, bv-s32-set!, bv-s64-set!): Unbox index and
  take unboxed S32 value.
  (bv-u8-ref, bv-u16-ref, bv-u32-ref, bv-u64-ref)
  (bv-s8-set!, bv-s16-set!, bv-s32-set!, bv-s64-set!): Likewise, but
  with unsigned values.
  (bv-f32-ref, bv-f32-set!, bv-f64-ref, bv-f64-set!): Use memcpy to
  access the value so we don't have to think about alignment.  GCC will
  inline this to a single instruction on architectures that support
  unaligned access.
* libguile/vm.c (vm_error_out_of_range_uint64)
  (vm_error_out_of_range_int64): New helpers.

* module/language/cps/slot-allocation.scm (compute-var-representations):
  All bytevector ref operations produce untagged values.

* module/language/cps/types.scm (define-bytevector-accessors): Update
  for bytevector untagged indices and values.

* module/language/cps/utils.scm (compute-constant-values): Fix s64
  case.

* module/language/tree-il/compile-cps.scm (convert): Box results of all
  bytevector accesses, and unbox incoming indices and values.

module/language/cps/utils.scm

@@ -223,7 +223,7 @@ disjoint, an error will be signalled."
           (($ $primcall 'scm->s64 (val))
            (let ((s64 (intmap-ref out val (lambda (_) #f))))
              (if (and s64 (number? s64) (exact-integer? s64)
-                      (<= (- #x8000000000000000) u64 #x7fffFFFFffffFFFF))
+                      (<= (- #x8000000000000000) s64 #x7fffFFFFffffFFFF))
                  (intmap-add! out var s64)
                  out)))
           (_ out)))