Add support for some serde serialization (#847)

* Add support for some serde serialization

cranelift/codegen/src/ir/entities.rs

@@ -22,6 +22,8 @@
 use crate::entity::entity_impl;
 use core::fmt;
 use core::u32;
+#[cfg(feature = "enable-serde")]
+use serde::{Deserialize, Serialize};
 
 /// An opaque reference to an extended basic block in a function.
 #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
@@ -103,6 +105,7 @@ impl GlobalValue {
 
 /// An opaque reference to a jump table.
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
 pub struct JumpTable(u32);
 entity_impl!(JumpTable, "jt");