Add unstable_extern_types feature

[madsmtm]

To see what changes we'd have to make in the library to use extern type (RFC-1861) when it's is stabilized.

This requires SSheldon/rust-objc-encode#7.

Unfortunately had to change some usage of ptr::null[_mut] to 0 as *const/mut X, and change the inner type of AtomicPtr, see rust-lang/rust#42847.

[SSheldon]

I'm very excited for extern types, thanks for doing this proof of concept! However I'm not excited about having another feature flag and a nightly only feature. I think we should wait for this feature to stabilize.

[madsmtm]

I think it's a good idea to have unstable features like these, since it gives nightly users (like myself 😉) the option to test it out on their code beforehand, and raise incentive for the feature to be developed on upstream. Besides, it isn't (or can at least be explicitly documented to not be) part of the SemVer contract since they're nightly only anyway - so theoretically, barring the changes to ?Sized bounds, this could be released in 0.2.8 and removed in 0.2.9 again if you wanted to.

Notable other places where the unstable feature thing is done:

• serde's unstable feature
• libc's const-extern-fn feature
• cargo itself does it
• Actually the whole "nightly" features idea in Rust follows mostly the same reasoning as I've given here