Support default debug_stub implementation for members that don't implement Debug

[jrmuizel]

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It would be nice to be able to add DebugStub and have all the members that don't have a Debug implementation automatically have a debug_stub value of "...". It seems like this should be achievable on rust nightly using specialization like the debugit crate does.

[kangalio]

Auto-deref based specialization might work too for this case, and it works on stable