Add serde support for `Slot` type (#10204)

Add serde support for the `Slot` type as we'd like to use it directly in our RPC.

substrate/Cargo.lock

@@ -9325,6 +9325,7 @@ version = "0.10.0-dev"
 dependencies = [
  "parity-scale-codec",
  "scale-info",
+ "serde",
  "sp-arithmetic",
  "sp-runtime",
 ]

substrate/primitives/consensus/slots/Cargo.toml

@@ -15,6 +15,7 @@ targets = ["x86_64-unknown-linux-gnu"]
 [dependencies]
 codec = { package = "parity-scale-codec", version = "2.2.0", default-features = false, features = ["derive", "max-encoded-len"] }
 scale-info = { version = "1.0", default-features = false, features = ["derive"] }
+serde = { version = "1.0", features = ["derive"], optional = true }
 sp-runtime = { version = "4.0.0-dev", default-features = false, path = "../../runtime" }
 sp-arithmetic = { version = "4.0.0-dev", default-features = false, path = "../../arithmetic" }
 
@@ -23,6 +24,7 @@ default = ["std"]
 std = [
 	"codec/std",
 	"scale-info/std",
+	"serde",
 	"sp-runtime/std",
 	"sp-arithmetic/std",
 ]

substrate/primitives/consensus/slots/src/lib.rs

@@ -24,6 +24,7 @@ use scale_info::TypeInfo;
 
 /// Unit type wrapper that represents a slot.
 #[derive(Debug, Encode, MaxEncodedLen, Decode, Eq, Clone, Copy, Default, Ord, TypeInfo)]
+#[cfg_attr(feature = "std", derive(serde::Serialize, serde::Deserialize))]
 pub struct Slot(u64);
 
 impl core::ops::Deref for Slot {