Add default as a serde dependency

Author: James Munns

Cargo.toml

@@ -39,6 +39,7 @@ docs = ["attributes", "unstable"]
 unstable = ["default", "broadcaster"]
 attributes = ["async-attributes"]
 serde-support = [
+  "default",
   "serde",
 ]
 std = [
@@ -74,7 +75,7 @@ once_cell = { version = "1.2.0", optional = true }
 pin-project-lite = { version = "0.1", optional = true }
 pin-utils = { version = "0.1.0-alpha.4", optional = true }
 slab = { version = "0.4.2", optional = true }
-serde = { version = "1.0.0", optional = true, features = ["derive"] }
+serde = { version = "1.0.0", optional = true, default-features = false, features = ["derive"] }
 
 [dev-dependencies]
 femme = "1.2.0"