Add advisory entries for futures-rs

[Qwaz]

I recently reported a Send/Sync bound error in futures-rs MappedMutexGuard (rust-lang/futures-rs#2239) and found few more soundness bugs in the futures-rs issue tracker. I don't have enough time to work on advisories for these bugs at the moment, so I'll leave the list here so that someone else can look into them.

• Add Send + Sync to ArcWake trait to fix unsoundness rust-lang/futures-rs#1654
• FuturesUnordered has an incorrect Sync impl rust-lang/futures-rs#2050
• noop_waker_ref is unsound rust-lang/futures-rs#2091
• Fix UB due to missing 'static on task::waker rust-lang/futures-rs#2206
• MappedMutexGuard Send/Sync bound is unsound rust-lang/futures-rs#2239

[BlackHoleFox]

The PR fixing a Send + Sync trait issue on ArcWake has a comment saying it was only present in pre-production releases and I confirmed that no currently available versions of the crate are available on crates.io missing the Send + Sync implementation on ArcWake. Does an advisory still need to be filed for it? Its been well over a year since.

[Shnatsel]

No. If it never made it into a release we have no machine-readable way to specify the version range. And anyone using development snapshots already threw caution to the wind, so we might as well not bother.

[Shnatsel]

@BlackHoleFox thanks for the high-quality advisories! Now that they're merged, I'm closing this.