Prefer variant structs over variant tuples

[jonhoo]

In a couple of places, enum variant tuples are used (e.g. MailboxDatum::List). It might be better if these were variant structs instead, so that their contents become self-documenting. For example, compare:

pub enum MailboxDatum<'a> {
    // ...
    List(Vec<&'a str>, &'a str, &'a str),
    // ...
}

with

pub enum MailboxDatum<'a> {
    // ...
    List {
        flags: Vec<&'a str>,
        delimiter: &'a str,
        name: &'a str
    },
    // ...
}

[djc]

Good point, done!