* Use rp2040-hal in all example (possibly through their bsp)
Some of the examples were using the cortex_m_rt::entry method which
misses the device specific spinlock re-initialisation.
This commits makes the usage more consistent by using rp2040_hal exported
macro as the only `entry` method used across examples.
* 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
* PWM functionality
* Updated prelude.rs
* Added example, cleaned up the PWM HAL.
* Renamed a file for clarity
* Changes to address C-CTOR recommendations, only 8 PWM channels, restructuring
* Forgot to remove a test function, and added a quick comment.
* Cleaned up code now that PWM channels are clustered in the PAC.
