Enable codegen-units=4 in debug mode

We previously had this and removed it because it caused runtime slowdowns
(discussion in #11102), but that was in release mode.

Debug mode is already very slow at runtime.
In my opinion, debug mode is mostly for the edit-compile-test cycle
which is currently dominated by compile times,
so sacrificing runtime for compile time makes sense.

I’ve observed a ~30% improvement of compile times with this.
This figure is consistent between a very fast desktop with 4 cores / 8 threads
and a not-as-fast laptop with 2 cores / 4 threads.

Cargo.toml

@@ -5,3 +5,12 @@ members = [
     "ports/servo",
     "support/android/build-apk",
 ]
+
+[profile.dev]
+codegen-units = 4
+
+[profile.release]
+opt-level = 3
+# Uncomment to profile on Linux:
+# debug = true
+# lto = false

ports/servo/Cargo.toml

@@ -35,12 +35,6 @@ testing = ["libservo/testing"]
 clippy = ["libservo/clippy"]
 debugmozjs = ["libservo/debugmozjs"]
 
-[profile.release]
-opt-level = 3
-# Uncomment to profile on Linux:
-# debug = true
-# lto = false
-
 [dependencies]
 backtrace = "0.2"
 browserhtml = {git = "https://github.com/browserhtml/browserhtml", branch = "crate"}