Split `DfConv` instance of AXI4 channels

[hiddemoll]

There are now two DfConv instances for AXI4: one for the combined write channels and one for the combined read channels. What we'd rather have is a DfConv instance for each channel. For each channel we should add DfConv, Simulate, Drivable and Test. This should make it easier to write AXI4 circuits and test them.

I've already done some work (see this branch):

• Added DfConv, Simulate, Drivable and Test instances of Axi4WriteAddress
• Added DfConv, Simulate, Drivable and Test instances of Axi4WriteData

However, I got stuck on the WriteResponse channel, which has data in the backwards direction instead of the forward one. In the bundled Axi4 DfConv implementation this was solved by using Reverse, but I'm not sure we can use that here.

I am stashing my work, because a refactor of the AXI4 protocol is probably imminent which would make adding DfConv (and friends) easier.