Get TinyUSB (hid and cdc devices) working

[auxym]

Just wanted to mention that I'm working on this, so effort doesn't get duplicated too much.

• [X] Get a basic example to compile with callbacks and tasks called from Nim
• [X] Send HID input reports from Nim (mouse / keyboard)
• [x] Send/receive CDC data from Nim
• [ ] Implement descriptor configuration (usb_descriptors.c file in tusb examples) in Nim
• [ ] Generate the tusb_config.h file from Nim (???) Not sure if this one is possible or worth the effort.
• [ ] Create a commented example
• [ ] Write (some) doc strings

WIP is here: https://github.com/auxym/picostdlib/blob/tinyusb/src/picostdlib/tusb.nim

I don't currently plan to work on other device classes (MSC, midi, audio, etc) or USB host.

[auxym]

So, I have HID and CDC working nicely, I even think the code is looking not too bad.

I looked at implementing usb_descriptors.c and it's just too macro-heavy, would have to be re-written from scratch as Nim macros, I'll leave that to someone better than me at macros (hm). tusb_config.h is a bunch of #defines that set options for tinyusb. Eventually I guess we could generate this from nim using templating (strformat or nimja etc), but for now I plan to leave these two files in C. They are "config" and don't implement any app logic.

So, my plan for now is clean-up an example and add docstrings where it makes sense, then submit a PR.