PWM support

[jonfin]

Add PWM support based on the stm32f4xx_hal implementation. It is mostly a copy from stm32f4xx_hal with some modification.
Only change to stm32f4xx_hal: it uses the recommended order for setting the register from DM00042534

I did some testing based on a stm32f030c8 chip.

fixes #26

[therealprof]

Thanks for the PR! I've tested it on a STM32F072 and it works a treat. If you could fix the CI issues and register the example with Cargo.toml I can merge this in.

[jonfin]

It should now work even on more devices. I missed the possibility that there could be other devices than the stm32f030 or stm32f070.

One thing is missing: the PWM implementation for TIM2 (32-bit). I skip it because I cannot test it after changing the calculation of the prescaler / Auto-reload value.

[therealprof]

Thanks, it's still missing the example registration in Cargo.toml. And if you could squash your PR a bit that'd be much appreciated.

[jonfin]

Is the example not automatic registered?

[therealprof]

Ah sorry, I missed that it's an example that works on all chips; not used to that. 😅

[jonfin]

Me neither ;)
I squashed the commits.

[therealprof]

LGTM, thanks.

bors r+

[bors]

Build succeeded:

• ci (stable)
• Rustfmt