fix(schema): handle parsing of "listpairs" in the DSL

Ref: ipld/ipld#281

Author: rvagg

.ipld

@@ -277,6 +277,8 @@ func spawnType(ts *schema.TypeSystem, name schema.TypeName, defn TypeDefn) (sche
 				return nil, fmt.Errorf("stringjoin has empty join value")
 			}
 			repr = schema.SpawnStructRepresentationStringjoin(join)
+		case typ.Representation.StructRepresentation_Listpairs != nil:
+			repr = schema.SpawnStructRepresentationListPairs()
 		default:
 			return nil, fmt.Errorf("TODO: support other struct repr in schema package")
 		}

schema/dmt/compile.go

@@ -467,6 +467,9 @@ func (p *parser) typeStruct() (*dmt.TypeDefnStruct, error) {
 			Join: join,
 		}
 		return defn, nil
+	case "listpairs":
+		defn.Representation.StructRepresentation_Listpairs = &dmt.StructRepresentation_Listpairs{}
+		return defn, nil
 	default:
 		return nil, p.errf("unknown struct repr: %q", reprName)
 	}