Handling of X1 X2 Y1 Y2 type lists in derive macro

[tgross35]

I'm thinking we could do something like

Struct Foo {
    #[from_record(defines_len = coords)]
    num_points: i32,
    #[from_record(array = (b"X", b"Y"))]
    coords: Vec<(i32, i32)>
}

And then our proc macro would:

• Use NumPoints to create a vector with defaults
• When it sees Xn or Yn, set that value in our vector