* 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
Previously the cargo configurations of all of the example projects had
`build-std = ["core"]`, which forces compilation of `core` as a
code-size optimisation. However, this is strictly unnecessary and will
currently break for users who do not use `rustup` directly (e.g. nix
users).
* Chips that have multiple cores will be exposed as chipname_corename,
i.e. stm32wl55jc_cm4
* Chips that have single cores will use the chip family as feature name
and pick the first and only core from the list
* Add support for stm32wl55 chip family
