Dynamic Topologies: step 1 (#17)

To facilitate dynamic execution topologies (what that means: being able
to add/subtract ALUs, branch units, FP units, etc), a Dispatch tester is
in order to ensure it's working properly.

This PR adds:

1. A Dispatch tester class
2. Generic Source/Sink classes for instruction groups or single
instruction
3. A more structured build environment
4. First cut of topology yaml files + core extensions