Review UART implementation

[190n]

Since UART only had an API review done, implementation review is still pending, and comments will be addressed in a v2.

[190n]

Ssince adding tests for the long transmits and receives with (clock, prescaler) other than (ISO, 4096), the tests show some spurious failures for Ben on board 0444000158dec8cf00000000000000000000000097969906, but not for Steven with the same board.