Use pattern matching to make Debug impl for Cfg more robust

[djc]

Follow-up from #3763:

Its nice to have the compile times improved (potentially - not measured), but this increases cognitive load when changes to the struct are made :(, so I'd really rather we solved that by choosing a non-syn backed crate, rather than removing the proc-macro entirely.

This change should help reduce cognitive overload when changes to the struct are made.

For me, crates like derivative also have cognitive load because their exact workings are kind of magic -- compared to a few lines of code doing a manual impl, that doesn't seem like a good deal to me.

[rbtcollins]

Thank you, this does address much of my concern. It will still require manual effort but at least it won't require thinking ;).

I wish Rust had a better story for compile time code execution, because I do totally get the its-magic aspect particularly for troubleshooting issues that arise when the generators go wrong.

OTOH, it is very convenient, and I've learned to embrace the magic much of the time :)

[djc]

Don't get me wrong, I love my procedural macros. I just think that all of the little derive utility crates generally add very limited value over manual implementations, which are pretty straightforward and entirely "in view".