Improve downgrade ergonimics

[Sh3Rm4n]

Rename downgrade to erase_pin or similar to conform to C-ERASED-PIN

[Sh3Rm4n]

nrf-hal names the generic pin Pin and not PXx. Also downgrade() has the name degrade() and has to be called only once to get the generic form, and not e.g.

let pin: PXx<Output<OpenDain>> = dp.gpioa.pa8.into_open_drain_output(gpioa.moder, gpioa.otyper).downgrade().downgrade()

Also:

How to change a generic pin from output to input? No method like into_open_drain_output() is implemented.
That is because we have to know which gpio bank the pin belongs to, to pass mutable access to the moder and otyper registers of that pin.