Support no_std + alloc

[val3rius]

Hi!

I'm currently running a local fork of this project in order to do some FFI things in a no_std application (with alloc). Of course it'd be a lot nicer to simply rely on the upstream repo.

If you take a look at the comparison against my fork (where I put all changes behind a no_std feature flag) you see its 90% just importing things from core rather than std. Would this be something feasible to get in libffi-rs?

master...val3rius:libffi-rs:no_std

Thanks a bunch!

[val3rius]

One problem to tackle would be that replacing std::process::abort(); with core::intrinsics::abort(); requires nightly. Perhaps there is some other way to go about that.