Thread-safe EGraph struct

[yihozhang]

We can make the EGraph implements the Send trait by substituting some of the components with thread-safe alternatives so this works:

    lazy_static! {
        pub static ref RT: Mutex<EGraph> = Mutex::new(EGraph::default());
    }

[codspeed-hq]

CodSpeed Performance Report

Merging #517 will not alter performance

_{Comparing yihozhang-thread-safe-egraph (61f5adc) with main (9e6ecb6)}

Summary

✅ 10 untouched benchmarks

[Alex-Fischman]

Fixes #516.