Add threading/parallel processing

It looks like a couple steps (namely control flow optimization, constants optimization, and C++ class hierarchy optimization) are bottlenecked by CPU (as opposed to memory). Per #51, everything is currently single threaded, which necessarily creates a bottleneck. For these steps in particular, is it viable to split tasks to be executed across multiple cores?

Start by locating the control flow optimization, constants optimization, and C++ class hierarchy optimization steps mentioned in the issue, then review the single-threaded design referenced by #51. Determine whether their tasks can be safely divided across multiple cores and identify the synchronization and correctness constraints. Done means a decided, validated approach to parallel processing for these bottlenecks.

Written by the indexing model from the issue text.