Consider adding specialized format_date fns to DateTimeFormatter

[sffc]

In #5269 (comment), we made the following conclusion:

pub struct Formatter<FSet>(FSet);

pub struct YMD;

struct Date;

impl<FSet> Formatter<FSet> {
    pub fn format() {

    }
}

// Stretch 1
impl Formatter<YMD> {
    /// See aliases module for more information
    pub fn format_date(x: Date) {}
}

// Stretch 2
type YMDFormatter = Formatter<YMD>;

This issue is to track progress on the stretch goals listed above.

[sffc]

Moving this to the backlog. If we see additional evidence that this would improve correctness or ergonomics, we can revisit.