Add access_banks flash accessor

[Abestanis]

This adds a access_banks helper to the FlashExt, which is a middle ground between access_bank1 and split.

Problem

Right now the only way to access bank 2 is using the split method, which consumes the FLASH instance. In my usecase I want to access both flash banks but I also need to flip the banks at some point, so I need to have access to the option control register via FLASH::optcr(&self), which needs the FLASH instance.

Solution

Using the new access_banks helper I can keep the FLASH instance around and get access to both banks as well as the option control register when I need it.

[richardeoin]

This looks useful, and I don't think it breaks any existing assumptions. After all it doesn't modify registers.

It would be interesting to know if similar crates (stm32-rs/stm32f4xx-hal or stm32-rs/stm32f7xx-hal perhaps) already have methods like this

[Abestanis]

• stm32-rs/stm32f4xx-hal's flash extension doesn't really distinguish between the two banks, they have a FlashExt::dual_bank(&self) -> bool, but their way of accessing the flash just takes a mutable self and returns an accessor that operates on sectors/offset from the flash base regardless of the bank: FlashExt::unlocked(&mut self) -> UnlockedFlash. So they don't have a split method and the unlocked method is like the access_banks method proposed here but it gives access to the entire flash range in one struct rather than two.
• stm32-rs/stm32f7xx-hal does not have a concept of banks, they just provide a thin wrapper over the FLASH peripheral with the fn lock(&mut self), fn program<'a, 'b>(&'a mut self, start_offset: usize, data: &'b [u8]) -> Result<ProgrammingSequence<'a, 'b>, Error> and fn unlock(&mut self) methods.

[richardeoin]

Nice, I appreciate for the summary. Given this crate's abstraction over the banks these methods definitely make sense. Thanks again for the contribution!