Support arbitrary::Arbitrary

[e00E]

num-bigint already supports quickcheck::Arbitrary which works roughly
like arbitrary::Arbitrary. arbitrary::Arbitrary used by cargo-fuzz and
makes it easier to use num-bigint BigInt and BigUint in projects that
want to fuzz that way.

https://github.com/rust-fuzz/arbitrary
https://github.com/rust-fuzz/cargo-fuzz

I have included an example fuzz target but I'm not sure whether we want to merge it.

cargo +nightly install cargo-fuzz
cargo +nightly fuzz run fuzz_target_1

[cuviper]

It's unfortunate that there are similar but separate Arbitrary traits, but I guess they have different models for generating data.

I'm OK with this, but can you add something to ci/test_full.sh to at least build with that feature?

[e00E]

I've added building the arbitrary feature to CI. I found the minimum rust version for arbitrary through testing with the Github CI.

Before merging we should still delete the fuzz folder. This was meant for reviewers but I don't think it makes sense in the repository.

[cuviper]

Before merging we should still delete the fuzz folder. This was meant for reviewers but I don't think it makes sense in the repository.

Ok, please do, and then I think we're good to go.

[cuviper]

bors r+

[bors]

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