streamer.

- when instantiating wr_transmission (top streamers), it is now possible
  to define that only RX or only TX or both (RX and TX) streamers are used.
  this is to save resources and still use all the other goodies added to
  the streamers and also wrpc/wr-cores (this configuration will be
  propagated to the top of board entity in the next commit. So, the
  user can still use the board (and even ref-design) and instantiate
  only reception (or tranmission) to save resources
- the statistics entity is now split into 3 entities (top, rx, tx)