#![no_std] Support #38

The Arbitrary crate isn't #[no_std], which prevent fuzzing #![no_std] crates. For now can be worked around by using #![cfg_attr(not(feature = "fuzzing), no_std)] at the top of the fuzzed crate instead of #![no_std], but it would be better not to need this workaround.

Although I don't really see a use case of doing the actual fuzzing in a #![no_std] environment, is would be nice not to need any reference to std in the fuzzed crate.

AFAIK, the arbitrary crate doesn't really rely on std features and pretty much everything required can be fetched from the alloc create.

I forked the crate and started to try to make the change. The first roadbump I encountered is the quote crate used in arbitrary-derive, so I might need to open a simillar issue over there.