adding a new show command, debug operation, and NoOpDestination

[tomreitz]

This PR adds two new features:

1. debug operation, to be used for debugging an earthmover project
2. earthmover show command, likewise to be used for debugging

as well as a few under-the-hood changes such as a new NoOpDestination (earthmover prunes graph branches with no destination attached, so I had to create this in order to be able to programmatically add a debug operation for earthmover show without actually materializing any file to disk).

I will go through and do a self-review, adding some comments about various parts of the changed code.