Support concurrent reading and writing via Uart

[SillyFreak]

Right now, the Uart struct has two methods

pub fn read(&mut self, buffer: &mut [u8]) -> Result<usize>
pub fn write(&mut self, buffer: &[u8]) -> Result<usize>

for receiving and sending data. As both of these methods require mutable access to the whole struct, reading and writing can't happen concurrently, although I think that should be conceptually possible. I've seen a few approaches that enable this:

• serial2::SerialPort only takes &self for reading and writing
• std::net::TcpStream has a try_clone() method
• tokio::net::TcpStream has split() and into_split() methods that each only give you access to one half of the stream

Personally I think the third option makes for the best API, because it prevents one from having multiple concurrent readers or writers respectively, but any way to enable this would be great!