Modbus on AVR64DA64

[T3sl4co1l]

I don't see any references to this, so here you go:
https://github.com/T3sl4co1l/freemodbus
The demo set is pretty extensive already, but I just added some AVR stuff to support AVR-DA and friends.  So, probably mega0, tinyAVR, etc.  Should be easier to add XMEGA now, too.

Also seems to be a very mature and widely used library.  Still testing, but it seems to work alright.  Not much trouble to integrate.  Seems to be around 3kB typical, not very heavy at all; though I have a few functions left to implement.  Doesn't look very efficient, in terms of operations, number of functions calls, etc. (in particular, callbacks from interrupts are very inefficient on AVR), but obviously, it's written for clarity / portability, and anyway, even if you're doing like 115.2kbps it's probably still a tiny performance cost to add to a project.

Tim