Auto merge of #21746 - llogiq:update-smallvec, r=emilio

This updates the smallvec crate and enables the union feature

We had a mix of 0.6.2 and 0.6.5 (which is the current release), this unifies to the latest version. It also enables the union feature which removes the discriminant, reducing memory usage.

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components/script/Cargo.toml

@@ -91,7 +91,7 @@ servo_geometry = {path = "../geometry" }
 servo-media = {git = "https://github.com/servo/media"}
 servo_rand = {path = "../rand"}
 servo_url = {path = "../url"}
-smallvec = "0.6.2"
+smallvec = { version = "0.6", features = ["std", "union"] }
 style = {path = "../style", features = ["servo"]}
 style_traits = {path = "../style_traits"}
 swapper = "0.1"