Add support for Box<dyn Format>

[Finomnis]

Rationale

I would like to be able to switch between a Locale and a SystemLocale at runtime. Therefore I would like to store them in Box<Locale> and Box<SystemLocale>, which both can be stored in a Box<dyn Format>.

Sadly, Box<dyn Format> is unsized and therefore unsupported by this crate.

As all the calls use &Format (as a reference), this can be fixed by adding + ?Sized to all function calls, allowing unsized object references.

This allows the following code to compile:

use num_format::{Format, Locale, SystemLocale, ToFormattedString};

fn main() {
    let locale: Box<dyn Format> = match SystemLocale::default() {
        Ok(sys_locale) => Box::new(sys_locale),
        Err(_) => Box::new(Locale::en),
    };

    println!("{}", 1_234_567_u32.to_formatted_string(locale.as_ref()));
}

1,234,567

[Finomnis]

@bcmyers This still maintained?

[Finomnis]

Already contained in #47, which also fixes other problems.

[Finomnis]

Actually, not completely contained in #47, so I'll keep this open