Constant time compare

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primitive ConstantTimeCompare
  fun apply[S: ByteSeq box = ByteSeq box](xs: S, ys: S): Bool =>
  """
  Return true if the two ByteSeqs, xs and ys, have equal contents. The time
  taken is independent of the contents.
  """
  if xs.size() != ys.size() then
    false
  else
    var v = U8(0)
    var i: USize = 0
    while i < xs.size() do
      try
        v = v or (xs(i)? xor ys(i)?)
      else
        return false
      end
      i = i + 1
    end
    v == 0
  end