Support `no_std` targets

[mkroening]

Summary of the PR

This adds no_std support by

• adding a default std feature for disabling impl Error on no_std.
• adding the Hashbrown dependency for no_std hash sets

Hashbrown also provides the HashSet implementation in std. std's HashSet is not in liballoc yet, though. By default, Hashbrown uses AHash as the default hasher. AHash is much faster than std's SipHash, but is not as HashDoS resistant. That seemed not relevant to me, though.

Closes https://github.com/rust-vmm/vm-fdt/issues/61

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [ ] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[mkroening]

I don't know how to modify CI to test this. Locally, I ran the following for testing:

cargo test
cargo test --no-default-features
cargo check --target x86_64-unknown-none --no-default-features

[andreeaflorescu]

I don't know how to modify CI to test this. Locally, I ran the following for testing:

cargo test
cargo test --no-default-features
cargo check --target x86_64-unknown-none --no-default-features

To test this as part of the CI you'll need to add a custom pipeline. Here is an example of a custom pipeline in another repository: https://github.com/rust-vmm/vm-virtio/tree/main/.buildkite. Once we get close with merging this PR, I can make the pipeline run on all PRs by configuring it in Buildkite.

[mkroening]

To test this as part of the CI you'll need to add a custom pipeline. Here is an example of a custom pipeline in another repository: https://github.com/rust-vmm/vm-virtio/tree/main/.buildkite. Once we get close with merging this PR, I can make the pipeline run on all PRs by configuring it in Buildkite.

I added a CI config. 👍

[andreeaflorescu]

I added the custom pipeline in Buildkite and retriggered the build. Any other PRs open against vm-fdt will fail the tests until this one is merged. If this becomes a problem I can also de-activate the custom build.

[mkroening]

Thanks :)

Sorry for dismissing the reviews. I force-pushed the fix for the CI.

[mkroening]

Thanks a lot! When can we expect a release? :)

[andreeaflorescu]

Thanks a lot! When can we expect a release? :)

I can just create one now.