num::cast(5.0).unwrap() is ugly

[brendanzab]

As mentioned in #319, num::cast(5.0).unwrap() is an ugly way for clients to construct generic literals. It might be worth exploring if cgmath can expose something nicer to authors of generic libraries.

[brendanzab]

So it seems like all floats support From<f32>. This would allow for things like S::from(1.0).

Unfortunately the integer situation is more complex. All integers From<i8>. All unsigned integers From<u8>. But coming up with a nice common conversion is hard, which makes it difficult to support parameterising by unsigned integers :(