As per Tokio and others, this commit provides a `poll_flush` method on `AsyncWrite` so that a best-effort attempt at wakening once all bytes are flushed can be made.
Based on the HAL from stm32wl, the peripheral driver has been
modified to fit into embassy, using the embassy APIs, providing
operation of the radio peripheral.
The initial version does not offer any async APIs, but the example
shows how the radio IRQ can be used to perform async TX of the radio.
* Add IRQ-driven buffered USART implementation for STM32 v2 usart
* Implementation based on nRF UARTE, but simplified to not use DMA to
avoid complex interaction between DMA and USART.
* Implementation of AsyncBufRead and AsyncWrite traits
* Some unit tests to ring buffer
* Update polyfill version
* Update sub module to get usart IRQ fix
