Remove RNG traits in favor of rand_core.

[jounathaen]

As proposed in #128 here is a PR that completely removes the RNG traits from the embedded-hal.
This closes #128.

Reasoning:

The rand_core crate already implements relevant traits for random number generation. A HAL which provides access to an RNG peripheral should implement these traits instead (Example: stm32f4xx-hal) and a driver should rely on these.

Impact:

This is a breaking change. Should be done before a 1.0.0 release.

Alternatives

• Don't drop this trait. See [Discussion] rand::RngCore for blocking RNG trait #128 for suggestions on this
• Instead of instantly dropping it, put in a deprecation warning and drop in a later point release. See Mark RNG traits as deprecated #271

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[jounathaen]

In the discussion, it was mentioned that some pointers and documentation on this change are required. I'm not sure on where to do this. The release notes seem to be the best place as it is hard to document stuff that is not existing anymore.

[jounathaen]

It's probably wiser to first mark the traits as deprecated as in #271 before removing them.

[eldruin]

Looks good to me. Thanks! I would go ahead with the removal without deprecation step.
Any objections @rust-embedded/hal?

[ryankurte]

i'm okay with it, maybe it'd be nice to have a note in the docs somewhere to go looking at rand_core?

[jounathaen]

Regarding the documentation: One could re-add the modules but without the traits and only with a note to rand_core.
I've already prepared the commit (but I've not added it to this PR yet to avoid commit spam).

[eldruin]

I would add the documentation note but not the rng module, since removing it would technically be a breaking change.

[jounathaen]

I would add the documentation note but not the rng module, since removing it would technically be a breaking change.

Sorry, but I didn't fully understand where you think the note should go.

[eldruin]

I think I would put it somewhere in lib.rs.