chore: Take down codegen-units in dev builds to 16 (#13466)

codegen-units determines how many object files are used when building a
single crate. By default it is set to 256 in dev builds and to 16 in
release builds. Higher values can get in the way of optimizations, but
they should help when performing an incremental build (as higher
granularity means that it's less likely we'd have to rebuild the whole
crate). When we were tinkering with Linux builds we found that we're
spreading ourselves too thin at times; large values of codegen-units
were making builds of smaller crates, such as file_finder, redundantly
long, where some CGs were miniscule. This PR significantly reduces the #
of CGs we use in dev builds. This means that an incremental build of a
crate might have to rebuild a bit more, but overall, we should be
spending *less* time in multicrate builds.

As a result of this change, incremental build of gpui (`cargo build;
touch crates/gpui/src/gpui.rs; cargo build`) goes down from 29-32s to
22s on my machine. Same scenario for editor: 13s to 11s. I've ran `cargo
clean` before executing each run



Release Notes:

- N/A

Co-authored-by: Thorsten <thorsten@zed.dev>

Cargo.toml

@@ -464,6 +464,7 @@ pathfinder_simd = { git = "https://github.com/servo/pathfinder.git", rev = "4968
 [profile.dev]
 split-debuginfo = "unpacked"
 debug = "limited"
+codegen-units = 16
 
 [profile.dev.package]
 taffy = { opt-level = 3 }