Use EFIAPI ABI introduced in latest nightly

[roblabla]

Fixes #38

Needs the very latest nightly (might not be released yet ^^).

[GabrielMajeri]

Thanks for the PR, I'll give it a few days to wait till the latest nightlies bring support for EFIAPI and then I'll merge it.

[GabrielMajeri]

Could you also update the docs to mention that people who use the entry API will need to add #![feature(abi_efiapi)] to their crate attributes for now?

[GabrielMajeri]

I've tested the PR on the latest nightly and the only thing missing is to also add #![feature(abi_efiapi)] to the test runner.

[roblabla]

Yup, I realized that. The other problem though (unless latest nightly fixes it) is that the rustfmt packaged with the nightlies is too old to handle the EFIAPI. So CI won't pass until rustfmt is updated.

EDIT: just checked, rustfmt of nightly 2019-11-01 stil doesn't like efiapi.

[josephlr]

I opened rust-lang/rustfmt#3903 to try to fix the rustfmt issue, but it requires updating some complex dependencies, so I think the best approach is to keep rustfmt running in the CI, but allow it to fail without failing the build.

Also, similar to r-efi/r-efi#15, this is a breaking change, so we'll need a 0.4 release for any version containing this PR.