Multisite: Introduce get_site()

Given a site ID or site object, `get_site()` retrieves site data in the same vein as `get_post()` or `get_comment()`. This will allow for clean retrieval of sites from a primed cache when `WP_Site_Query` is implemented.

Adds a `WP_Site::to_array()` method to support multiple return types within `get_site()`.

Props spacedmonkey.
See #35791.

Built from https://develop.svn.wordpress.org/trunk@37468


git-svn-id: http://core.svn.wordpress.org/trunk@37436 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-site.php

@@ -198,4 +198,16 @@ final class WP_Site {
 			$this->$key = $value;
 		}
 	}
+
+	/**
+	 * Converts an object to array.
+	 *
+	 * @since 4.6.0
+	 * @access public
+	 *
+	 * @return array Object as array.
+	 */
+	public function to_array() {
+		return get_object_vars( $this );
+	}
 }