Make the interface required by `SingleOutputPipe` similar to other
piperuns by requiring a constructor, the use of `PipeRunArgument`s and
the implementation of a non-static `run` function.
This commit is the final step in ensuring all the pipes commit what they
should. It also asserts this actually happens, enabling us to easily
catch future problems.
Replace the stub implementation of invalidation with the proper
implementation. A ReadPathCache is added to each global so that it can
keep tracks of what target are associated to which read paths.
Alessandro Di Federico