Vendor dependencies for 0.3.0 release

vendor/rustc-hash/README.md

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+# rustc-hash
+
+[![crates.io](https://img.shields.io/crates/v/rustc-hash.svg)](https://crates.io/crates/rustc-hash)
+[![Documentation](https://docs.rs/rustc-hash/badge.svg)](https://docs.rs/rustc-hash)
+
+A speedy, non-cryptographic hashing algorithm used by `rustc`.
+The [hash map in `std`](https://doc.rust-lang.org/std/collections/struct.HashMap.html) uses SipHash by default, which provides resistance against DOS attacks.
+These attacks aren't a concern in the compiler so we prefer to use a quicker,
+non-cryptographic hash algorithm.
+
+The original hash algorithm provided by this crate was one taken from Firefox,
+hence the hasher it provides is called FxHasher. This name is kept for backwards
+compatibility, but the underlying hash has since been replaced. The current
+design for the hasher is a polynomial hash finished with a single bit rotation,
+together with a wyhash-inspired compression function for strings/slices, both
+designed by Orson Peters.
+
+For `rustc` we have tried many different hashing algorithms. Hashing speed is
+critical, especially for single integers. Spending more CPU cycles on a higher
+quality hash does not reduce hash collisions enough to make the compiler faster
+on real-world benchmarks.
+
+## Usage
+
+This crate provides `FxHashMap` and `FxHashSet` as collections.
+They are simply type aliases for their `std::collection` counterparts using the Fx hasher.
+
+```rust
+use rustc_hash::FxHashMap;
+
+let mut map: FxHashMap<u32, u32> = FxHashMap::default();
+map.insert(22, 44);
+```
+
+### `no_std`
+
+The `std` feature is on by default to enable collections.
+It can be turned off in `Cargo.toml` like so:
+
+```toml
+rustc-hash = { version = "2.1", default-features = false }
+```