Require library users to decide on entry order criteria when packing

[phrohdoh]

ab256a1 added a TODO that this ticket covers.

The writer module's pack_directory function will want to take in a settings/options/configuration/etc. struct that contains a EntryOrderCriteria enum variant.

pub struct Settings {
    entry_order_criteria: EntryOrderCriteria,
}

pub enum EntryOrderCriteria {
    Length,
    Path,
}