Resolve most of the issues with config.rs

[cormacrelf]

Linkage: rust-lang#16254

Fixes:

1. Removes all of the duplication in the config_data! macro
2. Option::<T>::None everywhere by using $ty in the macro
3. Missing default snippet set
4. Serializing the unit values, the lack of which manifested itself as #[serde(untagged)] enums in an Option always rendering as null in the generated docs
5. Serializing true_or_always and false_or_never
6. cargo_unsetTest rendering differently

At this point, the generated documentation diff is down to zero, so all the tests pass.

You could probably remove the awkward @verbatim: prefix from the string-valued defaults if you made everything go back to the old system of parsing a string. It's up to you, I kinda like the type-based version. Notes:

• The string-based macro was good because it at least forced you to parse one of the values, which kinda acts like a test.
  • However you could just have the macro generate a test that does a ser-de round trip for every field. As long as you made a bunch of these types PartialEq. I did this in the branch test-roundtrip on my fork, of course it fails for the two fields that use string defaults.

[cormacrelf]

This is a bit of a hack. We may want three config sections, each being alphabetically sorted, and with a heading and a preamble. Or, alternatively, each documentation entry has an asciidoc tag saying which config class it is in, global/local/client.

[alibektas]

I thought about having the latter otherwise it is an extra step for users to figure out if the wanted config is a global/local/client.

[alibektas]

Thanks for the amazing work!