The cast to Level is incorrect, since the expression returns either 0 or
a positive value if the pin is set. The value is directly assigned to
the underlying uint8_t, meaning that Level::high won't match for levels
greater than 0.
Return a boolean and cast that to Level instead.
Remove the constructor from GPIO_pin so that we can use brace
initialization at compile time.
Rewrite the contents of pins.h to construct GPIO_pin types directly by
the use of a simple preprocessor macro.
Makes the code type-check and easier to read/extend.