Derive Arbitrary for multihash_codetable::Code

[expede]

I found myself writing a newtype wrapper over Code to get an impl proptest::Arbitrary, and thought "hey, why don't I upstream this one-liner?"

I tried to keep to your libraries and conventions as much as possible (e.g. arb), but please let me know if I missed anything.

(Some of the automated checks are failing, but I'm 90% sure those failures "shouldn't" be related to this change. It's late here, so I'll double check tomorrow)

[vmx]

The CI failures are unrelated, i've opened #367 for a fix.

[expede]

After #367, the tests got far enough to flag that there was a legitimate warning. However, I've now run into an edge case with the Cargo Hack action:

  50 | #[cfg_attr(feature = "arb", derive(arbitrary::Arbitrary))]
     |                                    ^^^^^^^^^^^^^^^^^^^^
     |                                    |
     |                                    unreachable call
     |                                    any code following this `match` expression is unreachable, as all arms diverge

I think this is a valid warning for the compiler to raise, but it only happens if none of the hash algorithms are enabled (which should never happen in normal use). The most pragmatic approach here is to allow the unreachable code (since it's useless without any hash algos anyway), so I did that. Please let me know if you'd prefer a different strategy. I wish that I could just allow the one derive, but it seems to not pick that up unless it's on the entire module (perhaps because it pertains to a macro?)

[vmx]

Thanks for the PR and figuring all the details out. The workaround the warning is totally fine.

[expede]

Thank you!