[Feature request] Split `SerialPort` into Writer, Reader and UsbClass

[Tnze]

It's usually people needs to write and read at the same time. Allowing split Writer and Reader or impl Write and Read on the immutable reference of SerialPort can make it happened.

It's also required by implement embedded-io-async that split UsbClass impl. Because when we .await on the SerialPort, no one can wake it because the ownership is borrowed by read(&mut self) and write(&mut self).