Make AxisDescription a non-tuple-struct

[jturner314]

I'm curious why AxisDescription is a tuple struct. Since the fields are pub anyway, I think it would be more convenient to change the definition to this

pub struct AxisDescription {
    pub axis: Axis,
    pub len: usize,
    pub stride: isize,
}

and remove the .axis(), .len(), and .stride() methods.

[bluss]

Sure and the methods can be deprecated first (there is no field-method name conflict)