Move schedule synthesis code to RealTimeScheduling.jl

[Ratfink]

The schedule synthesis code in src/schedule_synthesis.jl is fairly isolated in this package, and somewhat out of scope honestly. It's a pure real-time scheduling problem. The only non-scheduling piece of the file is in synthesizing constraints for controllers. That's important for us to keep around, but the rest of this file should probably find a new home.

I'd be happy to help move it to the RealTimeScheduling.jl package, where it fits better. Ideally the move would come along with support for that package's task and schedule types, possibly requiring some new conversion functions. I'm not in a rush on this, i.e. it can wait until after our upcoming deadline, but I do want to get this repository cleaned up more.