Vendor dependencies

Let's see how I like this workflow.

vendor/futures-core/README.md

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+# futures-core
+
+The core traits and types in for the `futures` library.
+
+## Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+futures-core = "0.3"
+```
+
+The current `futures-core` requires Rust 1.36 or later.
+
+## License
+
+Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or
+[MIT license](LICENSE-MIT) at your option.
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall
+be dual licensed as above, without any additional terms or conditions.