Add readme-sync

[daxpedda]

This makes sure our README and documentation in lib.rs are in sync. I also added a WRITE_README=1 environment variable to the test, so we can just update the README instead of manually syncing them by hand.

Remaining issues:

• Currently links are a bit broken, see Transform Links zheland/readme-sync#1, the workaround currently requires to specify links in lib.rs that aren't really necessary.
• Currently writing the README doesn't produce perfect output, see Write Output zheland/readme-sync#2.

[ModProg]

Running cargo test readme_sync twice with WRITE_README=1 fails on:

`Text("([#27]):")`
 does not match docs node
`Text("(")`
  --> README.md:12:1
   |
12 | ([\#27]):
   | ^^ ^^^^^^
   |
  ::: src/lib.rs/parsed
   |
10 |  ([#27]):
   |  ^
   |
  ::: src/lib.rs
   |
16 | //! ([#27]):
   | ------------

[daxpedda]

We will see how the issues I linked above get resolved, but I don't personally have time to do it myself.

I consider the linking issue blocking, the issue with weird output when writing can be ignored, the whole writing part can be removed, checking if both are in sync is still useful and works correctly.