Cargo.toml add backtrace to features

[RoDmitry]

backtrace was added to features (so it's visible on Crates.io, in versions). And also added all feature dependencies. So backtrace will automatically enable std, and std will enable backtrace/std feature (just in case if someday backtrace crate will remove std from the default features).

Also cargo check --no-default-features --features backtrace is working now. Maybe remove message:

#[cfg(all(feature = "backtrace", not(feature = "std")))]
compile_error! {
    "`backtrace` feature without `std` feature is not supported"
}

from build.rs?

[RoDmitry]

I have tried to make a backtrace feature visible on Crates.io, in versions. But now I understand. Just a wild guess) Never mind