Create newtypes for all PAC peripherals, mark unstable peripheral singletons

[bugadani]

Trying to break up #2956 into more manageable pieces :)

This change wraps all PAC types we use as peripherals in a newtype. This is a step toward removing the Deref implementation, and we'll no longer have to guess which of the peripheral::STUFF is a PAC peripheral, and which one is our newtype.

The PR also lets us hide some of the useless peripheral singletons - so far, only behind the unstable flag, but we should go one step further and introduce a private category for things like FE, or NRX.

[bjoernQ]

Maybe we could improve the docs a bit more

Stability here looks a bit weird

I see it's unstable - but I understood it the first time, already

[bugadani]

This is a consequence of using instability::unstable together with a public reexport - I don't plan on keeping the public reexports, so I hope this issue will go away.

[bugadani]

And indeed in the next PR the output looks like this:

[MabezDev]

LGTM, thanks!

[MabezDev]

Is marking the the top level module unstable not enough? Just for future me's understanding :)

[bugadani]

Something in the traits ends up not compiling because of some public reexport somewhere, it's a bit of a mess that I'm trying to untangle. (Specifically, without these we have Private type in public interface errors)