Refactor IWDG driver and add initialize function for prescaler with downcounter

[salkinium]

Ok, this escalated a little. I was just looking through the code and saw that the Iwdg class was not inlined, so I quickly refactored it and then noticed that the prescaler was manually computed and then I had to program a constexpr algorithm for it.

• [x] Inline IWDG functions
• [x] Move into modm::platform namespace.
• [x] Add prescaler + downcounter algorithm.
• [x] Add unittest for prescaler + downcounter algorithm.
• [x] Add Rcc::Lsi and Rcc::Hsi clock.
• [x] Add modm.digsep filter for nicely formatting long integers.
• [x] Add SystemClock::Iwdg to every board.
• [x] Add a generic assertBaudrateInTolerance implementation.
• [x] Add documentation for prescaler calculators.
• [x] Simplify tolerance assertion functions.
• [x] Added seconds_t, milliseconds_t, microseconds_t for template parameters.
• [x] percent_t can be float since C++20.
• [x] Fix minimum prescaler value for STM32 UART (8 or 16).
• [x] Implement iterator correctly so that the warnings go away. Help!

[salkinium]

Great, I messed up and merged this accidentally while rebasing. Fantastic…

[salkinium]

There's no way to open a merged PR, unlike closed PRs, so I'll address the review in a separate PR on the weekend. Damn it.