*  split into subpackages axi4 and axi4lite
*  moved each module into its subpackage
*  shortened names by removing Axi-prefixes wherever possible
*  moved Configs and IOs into corresponding companion object
*  replaced some printlns/printfs with Logging facilities

* moved config params into sealed case class
* unified constructors to use new config object
* added companion object for convenience constructors (backward
  compatible)
* changed constructor calls in unit tests accordingly

* addresses should only be handshaked when data is actually avaiable /
  needed; previous logic would provide addresses as fast as possible
* this fixes problems with handshaken addresses at other modules when
  adapter is being reset

* opposed to FifoAxiAdapter, addresses may only be supplied via the
  interface if data will be read (slave will supply data)
* this bug led to erroneous read bursts, overflowing the buffer
* fixed some minor condition issues

* replaced tick-tock-buffers with single FIFO
* burst size now independent of buffer size
* bursts start immediately when FIFO has space for one burst
* operation similar to FifoAxiAdapter
* unit tests work unchanged

* condition for switching of FIFOs led to 1-cycle delay
* now switching when either other FIFO empty, or dequeing
  in progress and exactly one element
* conditions are the same in both states

* now stays in fetch mode and just flips buffers, if current FIFO
  is empty
* empty checks more aggressive: now checks if FIFO will be empty
  in the next cycle to mask the state transition

* tick-tock buffer approach: one buffer is filled via AXI4 read
  bursts, the other is made available for dequeuing
* can provide continuous data at full speed, given that the bursts
  finish fast enough
* also added some unit tests for fixed blocks of memory
* missing: support for wrap-around (currently via reset)