440: I2c: simplify, expand docs, document shared bus usage. r=eldruin a=Dirbaio

~Depends on #441 -- check that one out first.~

This does some simplifications to the trait that I think we should do:

- Implement all methods in terms of `transaction`. This way HALs have to implement just that.
- Removed byte-wise-iteration methods: `write_iter` and `write_iter_read`. The reason is that they're quite inefficient, especially with DMA implementations. We've already removed these on other traits, so I think we should do as well here.
- Removed `transaction_iter`. I don't think it's much useful in practice, because the way iterators work all the yielded `Operation`s must have the same lifetime. This means that, even if the user can generate the `Operation`s on the fly, they can't allocate buffers for these on the fly, all buffers must be pre-allocated. So it's not useful for, say, streaming a large transfer by reusing some small buffer repeatedly. See #367 
- Removed useless lifetimes
- Standardized buffer names on `read` and `write`, I think they're clearer.

It also specifies how i2c bus sharing is supposed to work. This is an alternative to #392 . After the discussions there, I don't think we should split I2C into Bus and Device anymore. For SPI it makes sense, because drivers want to enforce that there's a CS pin (`SpiDevice`) or not (`SpiBus`). This is not the case with I2C, the API is exactly the same in the shared and non-shared case. Drivers shouldn't care which case it is.

So all we have to do to "support" bus sharing is docs, This PR does:

- Document that it's allowed for implementations to be either shared or not.
- Document some guidelines for drivers and HALs on how to best use the traits, expand the examples.


Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>