Implement Arbritrary for all types

[gluax]

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Building from the ground up with fuzzing in mind is always beneficial to libraries, especially ones that define their own types.

Meaning the Arbitrary trait should be implemented for all types, maybe even on the features themselves. This way, setting up libFuzzer, cargo fuzz, or AFL is easy.

The standard approach is to add a feature gate arbitrary that reveals the Arbitrary trait implementations when enabled.

[unbalancedparentheses]

I like this. Let's do it. Talking with the team