Make Register handling more elegant

[mre]

It might be possible to use ToString to get a register name instead of the RegisterId weirdness that I'm currently using.

Inspired by https://github.com/benashford/rs-es/blob/master/src/units.rs#L315-L329:

impl ToString for DistanceUnit {
    fn to_string(&self) -> String {
        match *self {
            DistanceUnit::Mile => "mi",
            DistanceUnit::Yard => "yd",
            DistanceUnit::Feet => "ft",
            DistanceUnit::Inch => "in",
            DistanceUnit::Kilometer => "km",
            DistanceUnit::Meter => "m",
            DistanceUnit::Centimeter => "cm",
            DistanceUnit::Millimeter => "mm",
            DistanceUnit::NauticalMile => "NM"
        }.to_owned()
    }
}