The HAL gpio_irq_api stores object IDs, which serve as a form of context
for the dispatch of the interrupt handler in the drivers level
InterruptIn Class. The way this is achieved is that the InterruptIn
Class casts its address to uint32_t, which is stored as the ID.
This results in compilation failure when the size of an object pointer
is greater than uint32_t, for example when building on a PC for unit
testing.

In order to allow Unit Testing of the InterruptIn Class, we replace the
use of uint32_t with uintptr_t (type capable of holding a pointer),
which allows portability and expresses intentions more clearly.
In aid of this latter goal, we also replace the use of the name "id"
with "context", to improve clarity - these are addresses of the context
related to that callback.