Add serde support for exposed types. (#40)

This adds derived `Serialize` and `Deserialize` implementations for
exposed types that describe registers, operands, and related program
inputs; entity indices; and regalloc output types. This allows
serialization of any of the embedder's IR data types that may embed or
build upon regalloc2 types.

These implementations (and the dependency on the `serde` crate itself)
are enabled only when the non-default `enable-serde` feature is
specified.

src/ion/data_structures.rs

@@ -584,6 +584,7 @@ impl PosWithPrio {
 }
 
 #[derive(Clone, Copy, Debug, Default)]
+#[cfg_attr(feature = "enable-serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct Stats {
     pub livein_blocks: usize,
     pub livein_iterations: usize,