Update cargo to 2024 edition (#842)

Some effects of this change:
- New formatting changes across many files. No functionality changes
should occur from that.
- Calls to `set_env` are considered unsafe, since this only happens in
tests we wrap them in `unsafe` blocks

codex-rs/Cargo.toml

@@ -16,6 +16,11 @@ members = [
 
 [workspace.package]
 version = "0.0.0"
+# Track the edition for all workspace crates in one place. Individual
+# crates can still override this value, but keeping it here means new
+# crates created with `cargo new -w ...` automatically inherit the 2024
+# edition.
+edition = "2024"
 
 [profile.release]
 lto = "fat"