Add storage ReadWrite trait, and helper structs, traits and iterators

[MathiasKoch]

I would like to propose an alternative solution to #28 .

There is currently one solution in #241, this is an alternative and simpler approach, highly inspired by the blog series by Cuervo in https://www.ecorax.net/as-above-so-below-1/ and https://www.ecorax.net/as-above-so-below-2/

The traits are implemented here (still working on the documentation): https://github.com/MathiasKoch/is25xp
and here: https://github.com/MathiasKoch/stm32l4xx-hal/blob/factbird-mini-1.0/src/flash.rs

I think this makes it MUCH easier for driver authors to make use of such a trait, compared to the solution given in #241 with a total of 5 traits. Also the helper iterators makes it much easier to handle optimizing for NOR flashes where writing 1s is not an option, without just defaulting to writing everything, as explained in above blog series.

EDIT:
After having used this for a while, there are a couple of notes and pain points to be addressed/discussed.

1. I think the try_erase function is close to useless as it is currently, as it does only allow erasing the full memory range, as opposed to sections/regions/range. Any suggestions for a better approach? One possible could be to take a range (fn try_erase(&mut self, range: (Address, Address)) or fn try_erase(&mut self, from: Address, to: Address))?
2. It might make sense to split the trait into a trait Read and a trait ReadWrite: Read, to support read-only memory? I have never encountered write-only, so i guess ReadWrite would make perfectly sense?

Any feedback is welcommed!

Closes #28

[rust-highfive]

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[MathiasKoch]

I am closing this in favor of https://github.com/rust-embedded-community/embedded-storage that we started to get things right and mature.