fix: add the missing unstable feature

[YuanYuYuan]

Closes #963. The change of feature flags only affects the dev-dependencies and is safe for the downstream dependencies/bindings.

[YuanYuYuan]

@Mallets can you please review this? Thanks.

[Mallets]

Self-reference in [dev-dependencies] created some problems in the past for release.
@fuzzypixelz @OlivierHecart could you provide more insight on this?

[fuzzypixelz]

Self-reference in [dev-dependencies] created some problems in the past for release. @fuzzypixelz @OlivierHecart could you provide more insight on this?

This has been extensively discussed in rust-lang/cargo#2911. @YuanYuYuan's workaround is standard procedure at this point. But it does introduce a cyclic dependency in the zenoh crate (to itself) which might break publishing: rust-lang/cargo#4242. Apparently Cargo checks if dev dependencies are published upon publishing. I'll have to test this.

[Mallets]

I believe #1131 solved this issue and it can be hence closed. @YuanYuYuan ?

[YuanYuYuan]

@Mallets, I've confirmed that the issue disappears after the fix. So let's close this PR.