Step-by-step VM guest debugging and GDB Server

[mitchellharper12]

Currently a WIP for first refactoring the VM runtime to support step-by-step debgging, to be followed by implementing the hooks to drive incremental state transition updates and reporting to consumers, followed by implementation of a GDB server for debugging VM guest code as a GDB target.

Creating this draft pull request so that progress can be tracked on the shape of the refactoring changes and API to ensure we are in agreement.