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.

.github/workflows/rust.yml

@@ -29,6 +29,14 @@ jobs:
     - name: Run tests
       run: cargo test --all --verbose
 
+  # Make sure the code typechecks with non-default features enabled.
+  features:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Check with all features
+      run: cargo check --all-features
+
   # Lint dependency graph for security advisories, duplicate versions, and
   # incompatible licences.
   cargo_deny: