cranelift: Implement serialize/deserialize for stack maps

When the `enable-serde` feature is set.

cranelift/codegen/src/bitset.rs

@@ -5,12 +5,14 @@
 //!
 //! If you would like to add support for larger bitsets in the future, you need to change the trait
 //! bound Into<u32> and the u32 in the implementation of `max_bits()`.
+
 use core::convert::{From, Into};
 use core::mem::size_of;
 use core::ops::{Add, BitOr, Shl, Sub};
 
 /// A small bitset built on a single primitive integer type
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[cfg_attr(feature = "enable-serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct BitSet<T>(pub T);
 
 impl<T> BitSet<T>