Add support for multi-threaded xpath evaluation

[maartendeprez]

The readonly module provides support for multi-threaded read-only operations over a parsed document. However, the findnodes method takes a shared reference to a Document, which is not readonly and therefore not Send + Sync. This makes it impossible, currently, to evaluate xpaths over the document from multiple threads.

This PR changes that by introducing a read-only version of Document, Context and Object, as well as a findnodes_readonly method on RoNode.

[dginev]

Thank you, this appears to be a very useful extension.

Could I also request that you mirror our one findnodes test here:

rust-libxml/tests/xpath_tests.rs

Lines 122 to 144 in 4224fd9

	#[test]
	/// Test that the dual findnodes interfaces are operational
	fn findnodes_interfaces() {
	let parser = Parser::default_html();
	let doc_result = parser.parse_file("tests/resources/file02.xml");
	assert!(doc_result.is_ok());
	let doc = doc_result.unwrap();
	// Xpath interface
	let mut context = Context::new(&doc).unwrap();
	let body = context.evaluate("/html/body").unwrap().get_nodes_as_vec();
	let p_result = context.findnodes("p", body.first());
	assert!(p_result.is_ok());
	let p = p_result.unwrap();
	assert_eq!(p.len(), 1);
	// Node interface
	let body_node = body.first().unwrap();
	let p2_result = body_node.findnodes("p");
	assert!(p2_result.is_ok());
	let p2 = p2_result.unwrap();
	assert_eq!(p2.len(), 1);
	}

With a similar entry inside readonly_tests.rs ?