Add transactional I2C interface

[eldruin]

An example where a transactional I2C interface would be an improvement is when facing the problem of sending an array of data where the first item is the destination address.
At the moment this requires copying the data into a bigger array and assigning the first item to the address. With a transactional I2C two write operations could be chained into one transaction so that it is possible to send the address and data without an extra copy.
This is specially problematic if the data length is unknown e.g. because it comes from the user.

You can see the problem here in the eeprom24x driver. In this case I am lucky enough to have the page upper limit so that the copy workaround is possible.

With this PR a bunch of code and macros could be replaced by doing something similar to this:

let user_data = [0x12, 0x34, ...]; // defined somewhere else. length may be unknown.
let target_address = 0xAB;
let mut ops = [
  Operation::Write(&[target_address]),
  Operation::Write(&user_data),
];
i2cdev.try_exec(DEV_ADDR, &ops)

I added a PoC here in linux-embedded-hal including an example of a driver where a classical combined write/read is performed through the transactional interface.

Note: A default implementation of the Transactional trait like in #191 is not possible because STOPs would always be sent after each operation. What is possible is to do is the other way around. This includes an implementation of the Write, Read and WriteRead traits for Transactional implementers.

This is based on previous work from #178 by @ryankurte and it is similar to #191

TODO:

• Add changelog entry

[rust-highfive]

r? @ryankurte

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[therealprof]

Doesn't the trait need an entry in the prelude?

[eldruin]

Doesn't the trait need an entry in the prelude?

Done.

[eldruin]

Is there anything left to do here? Since we have merged transactional SPI already, adding also transactional I2C to the same release would be nice.
I have also removed an entry about transactional SPI which was also present for the alpha.2 entry in the changelog.
However, #191 was only merged after the alpha.2 release.

[therealprof]

Excellent, let's get this in then.

bors r+

[bors]

👎 Rejected by code reviews

[therealprof]

... after @ryankurte removed his concern...

[therealprof]

bors r+

[bors]

Build succeeded:

• ci-linux (1.35.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)