GPIO input and outputs should be different types #39

Currently the pins always implement both InputPin and OutputPin even thought the pin might not be in the correct mode, and presumably trying to use it in the wrong mode either doesn't work or causes an error.

This could be done neatly with type states and into_output style methods like many other HALs. This would also make them more convenient to use as the user then doesn't have to manually set the correct mode before constructing the pin.