Correctly represent schema.StructRepresentation.

schema/type.go

@@ -151,8 +151,11 @@ var (
 
 type TypeStruct struct {
 	anyType
-	tupleStyle bool // if true, ReprKind=Array instead of map (and optional fields are invalid!)
-	fields     []StructField
+	// n.b. `Fields` is an (order-preserving!) map in the schema-schema;
+	//  but it's a list here, with the keys denormalized into the value,
+	//   because that's typically how we use it.
+	fields         []StructField
+	representation StructRepresentation
 }
 type StructField struct {
 	name     string
@@ -161,6 +164,21 @@ type StructField struct {
 	nullable bool
 }
 
+type StructRepresentation interface{ _StructRepresentation() }
+
+func (StructRepresentation_Map) _StructRepresentation()         {}
+func (StructRepresentation_Tuple) _StructRepresentation()       {}
+func (StructRepresentation_StringPairs) _StructRepresentation() {}
+func (StructRepresentation_StringJoin) _StructRepresentation()  {}
+
+type StructRepresentation_Map struct {
+	renames   map[string]string
+	implicits map[string]interface{}
+}
+type StructRepresentation_Tuple struct{}
+type StructRepresentation_StringPairs struct{ sep1, sep2 string }
+type StructRepresentation_StringJoin struct{ sep string }
+
 type TypeEnum struct {
 	anyType
 	members []string