Timer and Clock modules added

- Clock module to set and retrieve system clock which can have
  varying frequencies. Also allows enabling peripheral clocks
- Prelude updated
- Common time types added, based on stm32f1xx HAL implementation
- Basic timer implementation added

src/prelude.rs

@@ -2,3 +2,10 @@
 pub use embedded_hal::prelude::*;
 
 pub use crate::gpio::GpioExt as _va108xx_hal_gpio_GpioExt;
+
+pub use embedded_hal::digital::v2::InputPin as _embedded_hal_gpio_InputPin;
+pub use embedded_hal::digital::v2::OutputPin as _embedded_hal_gpio_OutputPin;
+pub use embedded_hal::digital::v2::StatefulOutputPin as _embedded_hal_gpio_StatefulOutputPin;
+pub use embedded_hal::digital::v2::ToggleableOutputPin as _embedded_hal_gpio_ToggleableOutputPin;
+
+pub use crate::time::U32Ext as _va108xx_hal_time_U32Ext;