Move all instances using transceivers to one TX domain (+whole bunch of bug fixes)

[martijnbastiaan]

In #688 we modified the transceiver logic to operate on a single TX domain (but multiple RX domains). To keep the changes contained, we still exported multiple clocks still (though just as a repeat). This PR cleans this up and removes all superfluous synchronization. During this process we discovered a number of bugs:

1. Transceiver up test would only fail if all links would fail. This is of course an error: it should fail if any link fails.
2. We had two fields called txReady (both on Input and Output). This caused confusion, that lead to the input's txReady to be looped back to the Output. This was clearly incorrect: Output.txReady should clearly depend on whether a neighbor says it is ready to receive data!

Fixes #696

TODO

• See if we can replicate handhshake failure in simulation test

[hiddemoll]

Discussed the only 2 comments I had with Martijn, who quickly fixed it.
Imo all changes make sense are the commit history clearly shows the process. I also know that the transceivers are really complex, and I don't have enough transceiver-knowledge to spot any errors in the current code.

If you found any potential issues in the transceiver code now, or other things you'd like to fix in the future, make issues now that they are fresh in your heads.

[martijnbastiaan]

Thanks :). Virtually all code was written by @leonschoorl, I just made the PR. It also has my seal of approval.

If you found any potential issues in the transceiver code now, or other things you'd like to fix in the future, make issues now that they are fresh in your heads.

We haven't found anything we didn't fix.