Doc tests

[BenjaminBrienen]

cargo test does not test code snippets in documentation because of how Cargo.toml is configured, so this ensures that documentation is written correctly. If this is detrimental, feel free to close this. I do think it would be a good enhancement, though. rustdoc expects code snippets to work unless told otherwise. Also note how many syntax errors were caught, such as fn main { or ($x:expr + $y:expr).

[ChayimFriedman2]

If we decide this is worth it, we should remove doctest = false from Cargo.tomls, not add another CI job for cargo test --doc,

[Veykril]

CI failure is #19214, given the proc-macro library changes CI runs nightly which hits that now

[lnicola]

You can rebase on top of master now.

[BenjaminBrienen]

The paths doc test fails on windows because /a/../../ is determined to be a relative path. I think this is incorrect. If I cd /sources on Windows, it changes the cwd to C:\sources.