Add Id::retain_autoreleased

[madsmtm]

An optimized version of Id::retain that uses objc_retainAutoreleasedReturnValue for when the previous caller returns an autoreleased value with objc_autoreleaseReturnValue. This is a common thing in ARC code to avoid unnecessary autorelease/retain pairs, but it is (apart from changing the retain count) purely an optimization.

This uses the newly stabilized inline assembly, which can easily cause very undefined behaviour if used incorrectly. Luckily, the instructions are exceedingly simple (essentially just a special nop), so there's almost no room for error - so fingers crossed I got this right (at least enough that it doesn't cause UB)!

I've tested this on my local machine (both x86 and x86_64), and I've also added assembly tests to ensure that enough optimizations actually happen to make it work. At some point I would really like a helper like Apple's TestRoot instead of the brittle checking of retainCount that we do now, but I'll defer that to later.

Related: gfx-rs/metal-rs#222

[madsmtm]

Since objc_retainAutoreleasedReturnValue, objc_retain, and so on return the same argument that they're given with, I briefly tested if it would be useful for LLVM to have this knowledge, encoded as follows.

if ptr != res {
    unsafe { core::hint::unreachable_unchecked() };
}

Conclusion: This was turned in to an @llvm.assume intrinsic, which can apparently have (and did have) detrimental impact on performance. So no.

[madsmtm]

I considered just changing Id::retain to use objc_retainAutoreleasedReturnValue, but decided against it, even though most of the time you want to use Id::retain_autoreleased. Primary reasons was: to stay closer to Objective-C naming, and because #120 hopefully will mean most users don't have to think about it.

[madsmtm]

Note: given that Cranelift doesn't support inline assembly, neither does rustc_codegen_cranelift yet, so that is broken.

A possibility would be to put the assembly stuff inside Id::retain_autoreleased behind a feature flag, I'm up for it if someone finds they need it.