Improve Debug implementation of Handle

[hannobraun]

By default, Handle has a very minimal Debug output:

println!("{global_vertex:?");
// prints:
// GlobalVertex @ 0x7f3e24305d40

Its Debug implementation now respects #, and will print full output if it is present:

println!("{global_vertex:#?");
// prints:
// GlobalVertex @ 0x7f3e24305d40 => GlobalVertex {
//     position: [
//         0.0,
//         0.0,
//         0.0,
//     ],
// }