This is going to be useful to implement continuous or asynchronous
one-shot modes.

`AdcExt` was mixed in between the `Adc` `impl` blocks.

I'm working on implementing other means of reading ADC values, so being
able to reuse this method will be useful.

This doesn't do anything in itself, as the ADC API is currently not
making use of this.

Add some crate-public methods that are useful when working with circular
DMA transfers.

Without support for hardware triggers, this API is of limited use, as
it's basically impossible to process the as fast as the ADC and DMA are
producing it.

Still, this puts in place a lot of infrastructure, and can be extended
to become more useful.

This change guards against some unexpected leftover configuration from
earlier interferring with the desired operation of the ADC.

It doesn't compile on the other targets.