I2c: simplify trait.

[Dirbaio]

Some simplifications 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 Operations must have the same lifetime. This means that, even if the user can generate the Operations 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.

[eldruin]

I think #440 is not too big and I reviewed that instead. I think we can close this one. Thank you!