chore(performance): Enable the "union" feature on SmallVec

This feature is not enabled by default.  From the docs: When the union
feature is enabled smallvec will track its state (inline or spilled)
without the use of an enum tag, reducing the size of the smallvec by one
machine word.

Signed-off-by: Bruce Guenter <bruce.guenter@datadoghq.com>

Cargo.toml

@@ -257,7 +257,7 @@ redis = { version = "0.21.3", default-features = false, features = ["connection-
 regex = { version = "1.5.4", default-features = false, features = ["std", "perf"] }
 seahash = { version = "4.1.0", default-features = false, optional = true }
 semver = { version = "1.0.4", default-features = false, features = ["serde", "std"], optional = true }
-smallvec = { version = "1", optional = true }
+smallvec = { version = "1", optional = true, features = ["union"] }
 snafu = { version = "0.6.10", default-features = false, features = ["futures"] }
 snap = { version = "1.0.5", default-features = false, optional = true }
 socket2 = { version = "0.4.2", default-features = false }