Bug 1716518 - Upgrade futures to v.0.1.31 and v0.3.15. r=emilio

Cargo.lock

third_party/rust/futures-0.1.29/.cargo-checksum.json

third_party/rust/futures-0.1.29/appveyor.yml

third_party/rust/futures-0.1.31/.cargo-checksum.json

third_party/rust/futures-0.1.31/CHANGELOG.md

third_party/rust/futures-0.1.31/Cargo.toml

third_party/rust/futures-0.1.31/LICENSE-APACHE

third_party/rust/futures-0.1.31/LICENSE-MIT

third_party/rust/futures-0.1.31/README.md

third_party/rust/futures-0.1.31/benches/bilock.rs

third_party/rust/futures-0.1.31/benches/futures_unordered.rs

third_party/rust/futures-0.1.31/benches/poll.rs

third_party/rust/futures-0.1.31/benches/sync_mpsc.rs

third_party/rust/futures-0.1.31/benches/thread_notify.rs

third_party/rust/futures-0.1.31/src/executor.rs

third_party/rust/futures-0.1.31/src/future/and_then.rs

third_party/rust/futures-0.1.31/src/future/catch_unwind.rs

third_party/rust/futures-0.1.31/src/future/chain.rs

third_party/rust/futures-0.1.31/src/future/either.rs

third_party/rust/futures-0.1.31/src/future/empty.rs