Add constructors for generated types.

[jamiebrynes7]

Currently, the only way to create generated types is to use the object initializer syntax (or at least that's what its called in C#). Instead, we should provide a constructor as well to cut down the verbosity.

Currently we have:

improbable::Coordinates { 
	x: 0.0, 
	y: 0.0, 
	z: 0.0 
}

where we could have:

improbable::Coordinates::new(0.0, 0.0, 0.0)

[randomPoison]

There's a nifty crate for making this easy to do, would allow us to write less code generation on our end: https://github.com/nrc/derive-new

[jamiebrynes7]

Hmmm - I wonder if its worth pulling in (and likely re-exporting) a dependency when it should be fairly trivial codegen to write?