Really the only interesting part is that serde_codegen now uses
synto parse the input instead of Syntex, which results in compile times for serde_derive similar to serde_macros (i.e. much faster than using serde_codegen directly).
In fact now that I actually try it, serde_derive compiles significantly faster than serde_macros. On my computer, from scratch including all dependencies it is 20 seconds for serde_macros 0.8.9 and only 12 seconds for serde_derive 0.8.10. The speedup is because syn also replaces our dependency on aster and quasi and quasi_codegen and quasi_macros.