Investigate why bincode deserialize is not threadsafe

[ludamad]

Caused the browser build to segfault
(However, given the lack of debugging tools we have for WASM, and the fact that it should not be just a WASM build issue, recommend debugging as below)

steps to fix:

• reintroduce parallel_for in PrivateExecutionSteps::parse(std::vector<PrivateExecutionStepRaw>&& steps)

for (size_t i = 0; i < steps.size(); i++) {
    ...
}

=>

parallel_for(steps.size(), [&](size_t i) {
    ...
});

• debug using cmake --preset tsan to instrument the code. This should reveal the offending line

[ledwards2225]

handled. (in part by moving to msgpack)