Implement Arbitrary optionally

[kvark]

Types of this crate are containers, which can be used in a variety of software. Some of that software needs to be fuzzed, and it's very convenient to do so via Arbitrary auto-derives. The particular example is https://github.com/gfx-rs/naga. We can work around this, but the suggestion still stands. It could be an optional dependency on arbitrary crate that derives that trait.