Vendor dependencies

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vendor/smallvec/README.md

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+rust-smallvec
+=============
+
+[Documentation](https://docs.rs/smallvec/)
+
+[Release notes](https://github.com/servo/rust-smallvec/releases)
+
+"Small vector" optimization for Rust: store up to a small number of items on the stack
+
+## Example
+
+```rust
+use smallvec::{SmallVec, smallvec};
+    
+// This SmallVec can hold up to 4 items on the stack:
+let mut v: SmallVec<[i32; 4]> = smallvec![1, 2, 3, 4];
+
+// It will automatically move its contents to the heap if
+// contains more than four items:
+v.push(5);
+
+// SmallVec points to a slice, so you can use normal slice
+// indexing and other methods to access its contents:
+v[0] = v[1] + v[2];
+v.sort();
+```