DigiCDCFast

Fork of the DigisparkCDC library from the Digistump AVR core 1.7.5 with the same SerialUSB API. The original waits 5 ms after every byte written, 1 ms before every USB poll and sends a zero-length packet after every data packet, which caps output near 200 bytes/s; without them it reaches 8000 bytes/s. Also fixes receive stalls, lost data and flush(), keeps the host's line settings (baud()), and offers DigiCDCMedium.h with 2-byte packets for timing-sensitive sketches. Since 1.1.0 about 450 bytes of flash smaller, with buffer sizes a sketch can choose and an optional hook to refuse line settings. A sketch can also borrow the USB interrupt at the end of each transaction (usbTransactionEnd), for work that cannot wait the 200 us a run of transactions takes, and answer vendor requests of its own beside the serial port (USB_CFG_VENDOR_HOOK), which is how one device can be a serial port to one program and something else to another without spending an endpoint. Needs the Digistump AVR core. Tested on Linux (PC, Raspberry Pi); Windows refuses low-speed USB serial devices.