Rust running with overflow-checks=false

Running Rust, a language that prides on being "safe", with overflow-checks=false, which is inherently an unsafe feature, is not representative of what the language stands for.

Also some of the Rust benchmarks run with `unsafe` sections. That is misleading and should be removed. It's not because the language allows that it can be used. It would be equivalent to running C++ with inline optimized assembly language.

Still, on a AMD 3960x the nsieve 12 benchmark runs at 226ms(gcc) and 246ms(clang) for C++ and 472ms for Rust, even with overflow checks turned off. That is very different from what the benchmark shows. Both compiled exactly with the build options on `bench/include/*`.