Queues: convert to Python library

[anshumanmohan]

Our queue frontend is getting larger and larger, and is totally polluting the calyx-py directory. Let us move it all into its own directory, and in doing so let us convert it into a Python library. Doing so will also make imports easier. In doing all this, let us make sure that the queues still get tested via runt. At present they get tested somewhat automatically because they live under calyx-py/test/correctness/....