Return more information for outlines

[ginnyTheCat]

Fix #68 by providing more information inside Outline by using the new LinkDestination struct and adding page_number to Location (which is what #68 is looking for).

The naming isn't great as there is page and page_number but i couldn't come up with anything better and that is what mupdf calls them themselves. I added a doc comment to clarify things. If somebody has a better idea for a name I would be more than welcome to change it.

This changes the output of .outlines() on a from, for example

Outline {
    title: "Copyright",
    uri: None,
    page: Some(
        0,
    ),
    down: [],
    x: 0.0,
    y: 0.0,
}

to

Outline {
    title: "Copyright",
    uri: Some(
        "OEBPS/Text/copyright.xhtml",
    ),
    dest: Some(
        LinkDestination {
            loc: Location {
                chapter: 23,
                page: 0,
                page_number: 186,
            },
            kind: XYZ {
                left: Some(
                    0.0,
                ),
                top: Some(
                    0.0,
                ),
                zoom: Some(
                    0.0,
                ),
            },
        },
    ),
    down: [],
}

[messense]

Feel free to merge after addressing minor issues.