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?