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.

Cargo.toml

@@ -16,6 +16,9 @@ smallvec = "1.6.1"
 fxhash = "0.2.1"
 slice-group-by = "0.3.0"
 
+# Optional serde support, enabled by feature below.
+serde = { version = "1.0.136", features = ["derive"], optional = true }
+
 # The below are only needed for fuzzing.
 # Keep this in sync with libfuzzer_sys's crate version:
 arbitrary = { version = "^0.4.6", optional = true }
@@ -37,3 +40,6 @@ trace-log = []
 
 # Exposes the internal API for fuzzing.
 fuzzing = ["arbitrary", "checker", "trace-log"]
+
+# Enables serde for exposed types.
+enable-serde = ["serde"]