Provide a way to spawn workflows that checks for topology issues

[mxgrey]

Currently it is possible to get a CircularCollect cancellation error when running a workflow if a user puts two collect operations in one cycle. This is only caught when a service is run, not when it gets spawned.

Another error that can happen when running a workflow is if the terminate operation cannot be reached from the workflow input.

Besides these two topological errors, all other mistakes or unsupported workflow constructions are caught by the compiler, making the need for a holistic workflow error check somewhat niche. It is also difficult due to the fact that the workflow is built by Commands, with each element put into a queue. The workflow Builder doesn't have a holistic view of what the user has constructed, making it difficult to scrutinize.

To implement this feature, we would probably need to change the Builder to internally keep track of the topology of what the user has built, and analyze this internal representation after the user has finished spawning the workflow. This has a risk of fragmenting the source of truth between the internal representation and the version that's queued up in the commands. Perhaps one way of solving this is to create a new World to initially spawn the workflow inside of, evaluate the workflow inside the self-contained world, and then use Commands to transfer the spawned workflow into the real world.