Use a single codegen-unit for compilation

as suggested in
https://nnethercote.github.io/perf-book/build-configuration.html#codegen-units

to achieve reproducible builds.
It can also enhance optimizations of the produced binary.

Fixes pop-os#230

This patch was done while working on reproducible builds for openSUSE, sponsored by the NLnet NGI0 fund.

Cargo.toml

@@ -38,6 +38,7 @@ serde_json.workspace = true
 serde_with.workspace = true
 
 [profile.release]
+codegen-units = 1
 lto = "fat"
 panic = "abort"