Add new async_context abstraction and refactor cyw43_arch to use it (#1177)

* Extract all poll/threadsafe_background/freertos from cyw43_arch into new abstraction async_context:
* provides support for asynchronous events (timers/IRQ notifications) to be handled in a safe context.
* now guarantees all callbacks happen on a single core.
* is reusable by multiple different libraries (stdio_usb can now be ported to this but hasn't been yet).
* supports multiple independent instances (independent instances will not block each other).
* cyw43_arch libraries cleaned up to use the new abstraction. Note each distinct cyw43_arch type is now a very thin layer that creates the right type of context and adds cyw43_driver and lwip support as appropriate.

Additionally,

* Add new pico_time and hardware_alarm APIs
* Add from_us_since_boot()
* Add alarm_pool_create_with_unused_hardware_alarm()
* Add alarm_pool_add_alarm_at_force_in_context()
* Add hardware_alarm_claim_unused()
* Add hardware_alarm_force_irq()
* Added panic_compact() and some minor comment cleanup; moved FIRST_USER_IRQ define to platform_defs.h

src/rp2_common/pico_bootrom/include/pico/bootrom.h

@@ -130,7 +130,7 @@ static inline void *rom_hword_as_ptr(uint16_t rom_address) {
 #endif
 
 /*!
- * \brief Lookup a bootrom function by code. This method is forceably inlined into the caller for FLASH/RAM sensitive code usage
+ * \brief Lookup a bootrom function by code. This method is forcibly inlined into the caller for FLASH/RAM sensitive code usage
  * \ingroup pico_bootrom
  * \param code the code
  * \return a pointer to the function, or NULL if the code does not match any bootrom function