NewUpdate builds an update with pages, and collects dependency information based on rules and other pages in its list.

Update represents a page number list, can be valid, fixed so that it is valid, can get page list, and middle page.

Dependencies implements sort.Interface for sort.Sort(sort.Interface) first index is the page #, and all following values are page #s this page should be before it also represents the current page number list, as it is a list of page numbers with their dependent page numbers.

Rules is a simple type to collect all pages that should follow a given page.