4th edition

For the fourth edition we have switched to the actual Chisel version 3.5.4.
We have added arbiter, priority encoder, and comparator to the Chapter of
combinational building blocks.
We have extended the hardware generation chapter with more functional
examples, including building a fair arbitration tree out of a simple 2 to 1
arbitration circuits.
We have added a new chapter in interconnect, bus interfaces,
and how to connect an IO device as a memory mapped device.
We have started a new chapter on debugging, testing and verification.
The plan is to extend the chapter on this important topic in the next edition.
We have extended the processor chapter with a more gentle introduction
of a microprocessor, including a figure of the datapath.