Buffered serial port interrupts

Mecrisp only implements the minimal serial interface required, i.e. USART1 with polled I/O. This is very limited, because the serial port has no buffering capability: if we don’t poll it often enough (over 10,000x per second for 115200 baud!), we risk losing incoming input data. The standard solution for this is interrupts: by enabling the RX interrupt, we can get the data out in time for the next one to be processed.