BufferedUart: Function to get number of bytes in buffer

[WesleyAC]

There should be a way to check the length of the UART tx and rx buffers, when working with BufferedUart.

[WesleyAC]

Oh, I just realized that the RingBuffer::len gets the (fixed) size of the ring buffer, and there's no function to get the amount of space used/free. I filed embassy-rs/embassy#4502 for that...