Flatten Uart module, remove unnecessary data, replace methods with apply_config

[bugadani]

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Pull Request Details 📖

Description

The main goal of this PR is to implement apply_config. This requires removing some internally tracked state, and moving some code around. cc #2416 and #1919

Some of the previous change methods weren't even public.

I'm also proposing flattening the module (i.e. removing config). When using multiple configurable peripherals, it's much neater to refer to the config struct as uart::Config than uart::config::Config. cc #2010

I'm also implementing embassy_embedded_hal::SetConfig because why not (and we'll want to implement it for Spi anyway, so the dependency will be used even if it's not terribly useful for UART).

[bjoernQ]

+1 for getting rid of the config module

[SergioGasquez]

LGTM! Thanks