Remapping flash traits to use base traits

[ryan-summers]

This PR reworks the NorFlash traits to use the Storage and ReadStorage base traits. That way, a library can accept a Storage or ReadStorage object without caring what the underlying storage type is.

I also added blanket &mut implementations for the Storage and ReadStorage traits.

[Dirbaio]

NorFlash is more restrictive: write() has "must erase first, must only write once after erase" restrictions. Storage is less restrictive, it models "just an array of bytes" so write() can write whatever at any time without caring about erases.

If a lib expects a Storage, it won't respect NOR flash restrictions, so passing it a NorFlash won't work.

We already have RmwNorFlashStorage to convert from NorFlash to Storage. It translates Storage::write() operations into the more restrictive NorFlash::write/erase(), reading an entire sector, modifying, erasing and writing it back if needed.

[ryan-summers]

This is a very good point that I hadn't considered.

RmwNotFlash seems like mostly what I want, but the downside is that the STM32 parts use a flash sector size of 128KB, so buffering a flash page on the stack blows up.

I wonder if we can consider another structure that doesn't preserve other contents in the page? But there's obvious risks to that as well.

[ryan-summers]

Closing this for the reasons outlined above. We may want to yank the ErrorType trait out as well, but there's no need for that right now.