Auto merge of #5603 - matklad:publish-nightly-features, r=matklad

Auto merge of #5603 - matklad:publish-nightly-features, r=matklad

Allow publishing crates with nightly features

closes #5427.

cc @rust-lang/cargo: I remember a vigorous debate over publishing crates with nightly Cargo features, but I can't recollect our exact plan of action. The discussion is logged here: https://paper.dropbox.com/doc/Unstable-Cargo-features-JBYMdsUYcO3FyW8Ubkjoz.

I think we just need to allow to publish crates with unstable cargo features, for the same reason we allow unstable rust features: you need explicit opt-in, even for deps. This is covered by Cargo tests: https://github.com/rust-lang/cargo/blob/9f097787b04b06cdde4fc42b26a531b22c1b37a6/tests/testsuite/cargo_features.rs#L115-L215.

I am not sure if we have ever implemented crates.io side of validation?