add a Debug impl for SharedShape

[whatf0xx]

Based on this issue in rapier: dimforge/rapier#577

I implemented Debug for SharedShape by using the trait method to get the ShapeType, and then using that as the basis of the debug info.

This is my first PR, I hope it's ok! I will post a corresponding one to rapier.

[sebcrozet]

Hi! Thank you for this PR. This is a great start.
It sounds more useful to use .as_typed_shape() instead of .shape_type(), that way you can print the actual geometry instead of just the type. Perhaps Debug could be implemented for TypedShape.

[whatf0xx]

Thanks for your feedback and sorry for the slow response, it's been a busy few weeks:)

I extended the #[derive(Debug)] to TypedShape and hence to all of its enum variants that didn't support it already. The only one which got a blanket/dummy implementation was TriMesh; I was confused because it threw an error trying to derive Debug for Qbvh here but elsewhere this wasn't an issue (for Polyline, for example). I would be happy enough to dig a little deeper and sort out a useful Debug implementation for TriMesh, too, but I'm not sure how useful that would immediately be, especially since the original issue was to just have a Debug implementation a few abstraction levels up...

[whatf0xx]

Hi! I checked that I can now build* with cargo build -p parry2d so this should pass the tests.

*with no warnings

[sebcrozet]

Thanks!