Implement Rng for Box with alloc feature

[pitdicker]

As discussed in #225 (comment).

I thought to run cargo test --no-default-features --feature=alloc, but that did not work. Endless compiler warnings.

I have included a couple a couple of commits that fix some of the tests that depend on needless memory allocations. Still about 40 compiler errors to go, most of which have to do with tread_rng not being available in lib.rs.

The test in lib.rs are where most of the remain problems are, and I didn't feel like fixing them at the moment. Those tests date from 5/6 years ago when rand.rs was the only file and things were very different. Several are now part of submodules, or could use a close look.

So testing on no_std does not work yet, but I hoped to leave that for another PR.

[dhardy]

I was a bit shocked by the size of this PR, but on closer look I like what you've done!

[dhardy]

I'd prefer if you put the use statement above, near #[cfg(feature="std")] use std::rc::Rc;.

[dhardy]

If you're going to tidy up, might as well just compare the sum here instead of average.

[dhardy]

Oh, yes, but is the test framework even available? I was wondering about cargo test --no-default-features but IIRC the unit test framework itself isn't even available. (Try commenting out all test code.)

[pitdicker]

That was my thought also, but I turns out to work. With everything commented out the PRNG tests run successfully.

[pitdicker]

I have replaced all uses of thread_rng() and ::test::rng() with ::test::rng(seed). I don't think it will win a beauty prize, but once we have better seeding and from_hashable we can make things better.

That took care of just about all errors left. Testing for no_std can be done with:

cargo test --tests --no-default-features
cargo test --tests --no-default-features --features=alloc

I did not replace thread_rng in the doctests. I don't want to use fixed seeds in the documentation, that would give the wrong impression. So doctests can only run with the std feature.

[pitdicker]

(I rebased the first 5 commits, I believe a documentation change in chacha made it unmergable. But otherwise didn't touch them)

[dhardy]

Nice work!

I agree, I don't think it's worth changing the doc-tests.

One final thing to do: add those new tests to CI.