* Use RUSTFLAGS to set the optimization level
Cargo has a [quirk]() in how configuration settings are propagated when `cargo test` runs: local code respect the settings in `[profile.test]` but all dependencies use the `[profile.dev]` settings. Here we force `opt-level=3` for all dependencies.
* Remove unused profile settings
* Maybe like this?
* Turn off incremental compilation
* Remove colors; try again with overflow-checks on
* Use quiet CI machine
* Turn overflow checking back on
* Be explicit about what options we use
* Remove "quiet machine" override
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