* implemented both hardware (runtime) logging and constructor logging
  (compile time)
* mix-in trait Logging provides methods info, warn and error (and cinfo,
  cwarn, cerror resp. for constructor logging)
* logging level is controlled by implicit Logging.Level, determined at
  each call, allows for selective logging

* replaced unit test with much more generic property specs
* also refactored test classes into module-subpackages

* defined generic property for DecoupledDataSource:
* testing correctness (correct order + value of data), handshakes (wait
  while consumer is not ready) and repeat behavior
* running random tests over arbitrary bitwidth, size and repeat flags