USB support for Leonardo

[supersimple33]

I have take the code which was written by @agausmann here and turned it into this PR with a number of modifications so that it is hopefully useful to programmers working at every level of this repository.

At the generic level I have added a new macro (v2) to make a AvrGenericUsbBus struct which implements UsbDevices UsbBus trait. Then at the mcu hal I have called this macro and then also provided it with a SuspendNotifier struct and implemented a new trait ClearInterrupts for necessary interrupt registers. I also define mcu specific constants here as well. At the arduino hal level I import all the important parts from the mcu hal level and then also supply two convenience macros for creating parts of the USB setup. Finally I have added the code for a new example which uses all of this code and follows the demo keyboard as set out in the original atmega-usbd.

This addresses #40.
I have also included some code for the atmega8u2 in anticipation of #568.

[supersimple33]

I won't have access to my leonardo for a few weeks and haven't been able to test these changes which is why I marked as draft. If someone can confirm they work which I don't see why they wouldn't then I'll open for review.

[rattboi]

I can confirm that this seems to be working as expected. I built avr-hal against his PR and used his keyboard example on an ItsyBitsy 32u4, pretending it's a Leonardo.

After flashing with ravedude, a new USB HID keyboard device showed up.

setting d2 low triggers the payload and you can see the hello world string in my terminal.

[1579390.792130] usb 1-1: USB disconnect, device number 10
[1579391.999568] usb 1-1: new full-speed USB device number 11 using xhci_hcd
[1579392.169033] usb 1-1: New USB device found, idVendor=1209, idProduct=0001, bcdDevice= 0.10
[1579392.169048] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[1579392.169054] usb 1-1: Product: Bar
[1579392.169058] usb 1-1: Manufacturer: Foo
[1579392.190625] input: Foo Bar as /devices/pci0000:00/0000:00:08.1/0000:c1:00.3/usb1/1-1/1-1:1.0/0003:1209:0001.003A/input/input90
[1579392.243546] hid-generic 0003:1209:0001.003A: input,hidraw9: USB HID v1.10 Keyboard [Foo Bar] on usb-0000:c1:00.3-1/input0
[1579392.325168] input: Foo Bar as /devices/virtual/input/input91

ratttop :: code/avr-rust/avr-hal-test ‹main*› % HelloHello WorldHello WoHello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello WorldHello World