Experiment with optimizations for build dependencies
Gecko's Cargo.toml
has this nugget:
# Optimize build dependencies, because bindgen and proc macros / style
# compilation take more to run than to build otherwise.
[profile.dev.build-override]
opt-level = 1
[profile.release.build-override]
opt-level = 1
Maybe that would make it faster to build our dependencies that use proc macros?