Add trait embedded_io_async to uarte

[wackazong]

No description provided.

[Dirbaio]

The raw Uart can't impl embedded_io::Read for a few reasons:

• It is is unbuffered, this means it can only receive while the task is actively waiting on a .read().await call. Data that's received between read calls is lost. The embedded_io::Read contract requires that data is not so easily lost, drivers using it can assume that.
• It must return early if less than buf.len() bytes are received. Drivers can call read() with a buffer bigger than the data they're expecting (for example, if they don't know the size of the data in advance) and get back a partially filled buffer with the data that arrived and process that. With this impl, they'd hang instead.

for embedded_io::Read there's the buffered uart which does extra fancy things to fulill the embedded-io contract.

Could you add just the Write impls?

[wackazong]

Thanks for the explanation. I changed it as requested.