cgo: refactor union support

Instead of putting the magic in the AST, generate regular accessor
methods. This avoids a number of special cases in the compiler, and
avoids missing any of them.

The resulting union accesses are somewhat clunkier to use, but the
compiler implementation has far less coupling between the CGo
implementation and the IR generator.

compiler/interface.go

@@ -210,10 +210,6 @@ func getTypeCodeName(t types.Type) string {
 		return "slice:" + getTypeCodeName(t.Elem())
 	case *types.Struct:
 		elems := make([]string, t.NumFields())
-		if t.NumFields() > 2 && t.Field(0).Name() == "C union" {
-			// TODO: report this as a normal error instead of panicking.
-			panic("cgo unions are not allowed in interfaces")
-		}
 		for i := 0; i < t.NumFields(); i++ {
 			embedded := ""
 			if t.Field(i).Embedded() {