Make internal BigMath method a private method

[tompng]

private def has no effect in BigMath module. Change it to private_class_method

module BigMath
  module_function

  # Private BigMath#foo and public BigMath.foo
  def foo; end 

  # Same as above. private has no effect
  private def foo; end

  # Private BigMath#foo and private BigMath.foo
  private_class_method def foo; end
end