* Improve clock frequency stuff for uninitialized clocks
- Made clocks singletons so the frequency handling actually works as expected
- Added initial frequencies
- Improved the docs
- Added a Clock trait
* Add pico examples.
These have the benefit of knowing which external crystal is attached.
Even though it always should be a 12 MHz crystal.
Thus we can setup the clocks properly
I also changed the rp2040 examples to work out of the box for pico boards since that will probably be used most of the time
