Some code restructuring

 This is quite a bit in one commit but it:

       * Splits the app into an app and module so the analysis components
         can be re-used. Also tidied up all the header files in the app.
       * Updates the app_handler to syphon off i2s samples (renamed to i2s_tap)
       * Changes the communication between the i2s tap and the analyzer
         slightly
       * Updates to be able to test based on i2s loopback

   Currently the code only works in simulation (axe command line added to
   Makefile) but we should be able to get hardware working by setting the correct ports
and doing the correct codec config for the board.