vcd: handle enums with fields

crates/fayalite/src/sim.rs

@@ -1428,7 +1428,7 @@ impl Compiler {
                             index: discriminant,
                             ty,
                         }),
-                        name,
+                        name: "$tag".intern(),
                         ty,
                         flow,
                     };

crates/fayalite/src/sim/vcd.rs

@@ -491,7 +491,21 @@ impl WriteTrace for TraceArray {
 
 impl WriteTrace for TraceEnumWithFields {
     fn write_trace<W: io::Write, A: Arg>(self, writer: &mut W, arg: A) -> io::Result<()> {
-        todo!()
+        let arg = arg.in_type();
+        let Self {
+            name,
+            discriminant,
+            non_empty_fields,
+            ty: _,
+            flow: _,
+        } = self;
+        write_vcd_scope(writer, "struct", &name, |writer| {
+            discriminant.write_trace(writer, arg)?;
+            for field in non_empty_fields {
+                field.write_trace(writer, arg)?;
+            }
+            Ok(())
+        })
     }
 }