The separate Schmitt trigger bit in the configuration is dropped, as
the Schmitt trigger is only every disabled when in `GPIO_DISCONNECT`
mode. So no need to encode the same information twice.
The `gpio_state_t` is improved to be a bitmask that holds the
MODER register value and a flag indicating whether open-drain mode
should be enabled.
Finally, `GPIO_DISCONNECT` is implemented. This is done by placing the
GPIO in analog mode, which by disabling the Schmitt trigger reduces
power consumption.
6028097132