async: provide wrapper adapter for sync impls

[jordens]

It would be nice to have a little adapter like this.

use embedded_storage::nor_flash::ReadNorFlash as SyncReadNorFlash;
use embedded_storage_async::nor_flash::ReadNorFlash as AsyncReadNorFlash;

pub struct NorFlash<F>(F);

impl<F: SyncReadNorFlash> AsyncReadNorFlash for NorFlash<F> {
...
}