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/lib.rs

@@ -2,6 +2,10 @@
 
 pub use va108xx;
 
+pub mod clock;
 pub mod gpio;
 pub mod prelude;
+pub mod time;
+pub mod timer;
+
 pub use va108xx as pac;