Support Node#pointer_id

[takahashim]

This PR adds Nokolexbor::Node#pointer_id, which returns the underlying C node pointer as an Integer.

Example

require "nokolexbor"

doc = Nokolexbor::HTML("<div id='x'><p>hi</p></div>")

n1 = doc.at_css("#x")
n2 = doc.at_css("#x")

n1.pointer_id == n2.pointer_id   # => true
n1.pointer_id.class              # => Integer

Compatibility

This PR adds Node#pointer_id, matching Nokogiri::XML::Node#pointer_id.
Both return an Integer derived from the underlying C node pointer, so the same C node yields the same pointer_id.

[zyc9012]

Thank you @takahashim 👍 LGTM!