Add 'as_imperial()' function / type for nice formatting.

[thejpster]

I would like to see the following work:

let d = Distance::from_metres(100);
println!("{}", d.as_imperial()); // gives 109 yards, 1'1"

Will require fn as_imperial(&'a self) -> DistanceImperial<'a>, where DistanceImperial just holds a reference to the parent, and impl's Display. You could also have a metric version as_metric() which only gives one unit, but selects the appropriate SI scale (e.g. 33.3km, 33.3m, 33.3cm, 33.3mm, 33.3μm, etc).

[thejpster]

Selecting of SI scales now happens through get_appropriate_units. I'm still interested in as_imperial though, which would select an appropriate imperial unit (or collection of imperial units, e.g. 14 miles, 1200 yards, 3', 2¼")