Facade Links

Links used to be redundantly identified: There could be only one link between a combination of Node/Gate and Node/Slot, but links had randomly generated UIDs nonetheless. Also, there was a central dict of all links that was excessively used by the whole of the /nodenet codebase, even when local access to links (through gates and slots) was possible.
To reduce redundancies and prepare for alternative implementations of the nodenet interfaces, link management, with these changes, has been pushed back from runtime/nodenet to nodes, and links are referenced by their node/gate/slot/node combination instead of by random UIDs.
UIDs are still being generated transparently, but only for the convenience of clients that still use them.
We should later remove link UIDs entirely and leave it to clients whether they want to generate UIDs for their own purposes.