Auto merge of #7956 - camsteffen:author, r=llogiq

Author improvements

changelog: none

Various aspects of the author implementation are re-imagined to be much less repetitive. Also fixes some bugs. I hope this makes author more fun to work on for future contributors.

The last commit is pretty heavy but I tried to at least separate some changes so that the test file diffs per commit are simple.

Author: bors

clippy_lints/src/lib.rs

@@ -2,6 +2,7 @@
 
 #![feature(box_patterns)]
 #![feature(drain_filter)]
+#![feature(format_args_capture)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]
 #![feature(once_cell)]