Trial causal profiling

I've long wanted to use https://github.com/plasma-umass/coz in a project, to more easily see where to spend optimization time. Since fontmake-rs is a new effort, it might pay to put it in early? The Rust port is at https://github.com/plasma-umass/coz/tree/master/rust.

Start with the linked coz repository and its Rust port to understand how causal profiling could be tried with fontmake-rs/fontc. Determine the integration scope and what profiling output would demonstrate value; done means a documented trial produces usable optimization guidance.

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