Add support for the most widely used STM32F1xx series MCUs

[wuwbobo2021]

STM32F1 is probably the most popular series in ST’s product line for many years. Is there any plan about it?
There’s another crate stm32f1xx-hal, lacking some functions (especially those for drving TIMs), and its development progress seems slower than stm32f4xx-hal, why is this the case?

[David-OConnor]

No. This lib focuses on newer series, to limit scope and complexity, and make it more manageable to maintain. The F1 series is, IMO, not a good choice for a new device design, as there are a number of newer series that have subtle improvements.

For example, for most things f1 does, I would recommend g0 or similar for a new device. If you are supporting a legacy device, it's almost certainly not using this lib!