Improve construction of common quantities

[unexcellent]

I often have to construct quantities outright for simulation and I think its more verbose than it should be, especially for very common units. I have implemented a trait in my own repos that can convert the standard numerical types into lengths like this

assert_eq!(
    5.cm(),
    Length::new::<centimeter>(5.)
);
assert_eq!(
    16.4.ft(),
    Length::new::<foot>(16.4)
);

(have a look at this for how it is implemented)

I have asked @iliekturtles if this would be something that could be implemented into uom directly and this was his response:

I think a trait like this, possibly implemented behind a feature gate, would be useful. Please go ahead and open an issue. A PR to implement is also very welcome.

At a high level I'm thinking a To$Quantity trait could be implemented in https://github.com/iliekturtles/uom/blob/master/src/quantity.rs with a method for each unit. I've skipped over the complications of the U and V generic parameters, but believe we should be able to work through these.

fn $unit(self) -> $quantity<...> {
$quantity::new<$unit>(self)
}

So that is why I am opening this issue. I will try to implement it myself this or next week and open a PR if I manage.