lib: derive Debug on StartHandshake

[lucab]

This auto-derives a Debug implementation for StartHandshake.

[lucab]

I also wanted to enable the missing_debug_implementations lint for all public types, but I found that LazyConfigAcceptor needs some massaging first.
I opted for tweaking rustls with rustls/rustls#2592 for now, deferring the lint for later.

[djc]

I'm not that convinced. These aren't long-lived types nor are the debug values seemingly going to be especially meaningful.

[lucab]

@djc that's a fair point.
The (only, so far) type where I have encountered some friction is StartHandshake, which I'm storing as part of a state machine.
I can make the surface of this PR smaller (just that type?), or abandon it at all and implement my custom Debug in my consumer. Let me know what you prefer.

[djc]

I can make the surface of this PR smaller (just that type?), or abandon it at all and implement my custom Debug in my consumer. Let me know what you prefer.

I'd be okay reviewing another iteration that only adds Debug for that type.

[lucab]

Thanks for the feedback! I took the occasion to also add a docstring (and some breadcrumbs) on that type.

[cpu]

Thank you!