Add norman field attribute "pointer"

Add a new field attribute "pointer" to indicate that the generated
client code for the field must be a pointer. This allows clients to
differentiate between sending nil/leaving the value unset and sending an
empty map or slice.

This change also removes the `nullablestring` norman type introduced in
30f8d18 since schemas that need a pointer to a string can now use this
field attribute. There are no libraries currently using this feature so
it should be safe to remove.

Example usage:

```
Labels map[string]string `json:"labels" norman:"pointer"`
```

Resulting API schema:

```
"labels": {
  "create": true,
  "nullable": true,
  "pointer": true,
  "type": "map[string]",
  "update": true
}
```

Generated client code:

```
Labels *map[string]string `json:"labels,omitempty" yaml:"labels,omitempty"`
```

generator/generator.go

@@ -35,17 +35,19 @@ type fieldInfo struct {
 }
 
 func getGoType(field types.Field, schema *types.Schema, schemas *types.Schemas) string {
-	return getTypeString(field.Nullable, field.Type, schema, schemas)
+	return getTypeString(field.Nullable, field.Type, field.Pointer, schema, schemas)
 }
 
-func getTypeString(nullable bool, typeName string, schema *types.Schema, schemas *types.Schemas) string {
+func getTypeString(nullable bool, typeName string, pointer bool, schema *types.Schema, schemas *types.Schemas) string {
 	switch {
+	case pointer:
+		return "*" + getTypeString(nullable, typeName, false, schema, schemas)
 	case strings.HasPrefix(typeName, "reference["):
 		return "string"
 	case strings.HasPrefix(typeName, "map["):
-		return "map[string]" + getTypeString(false, typeName[len("map["):len(typeName)-1], schema, schemas)
+		return "map[string]" + getTypeString(false, typeName[len("map["):len(typeName)-1], false, schema, schemas)
 	case strings.HasPrefix(typeName, "array["):
-		return "[]" + getTypeString(false, typeName[len("array["):len(typeName)-1], schema, schemas)
+		return "[]" + getTypeString(false, typeName[len("array["):len(typeName)-1], false, schema, schemas)
 	}
 
 	name := ""
@@ -81,8 +83,6 @@ func getTypeString(nullable bool, typeName string, schema *types.Schema, schemas
 		return "string"
 	case "hostname":
 		return "string"
-	case "nullablestring":
-		return "*string"
 	default:
 		if schema != nil && schemas != nil {
 			otherSchema := schemas.Schema(&schema.Version, typeName)

parse/builder/builder.go

@@ -382,8 +382,6 @@ func ConvertSimple(fieldType string, value interface{}, op Operation) (interface
 		return convert.ToString(value), nil
 	case "reference":
 		return convert.ToString(value), nil
-	case "nullablestring":
-		return convert.ToString(value), nil
 	}
 
 	return nil, ErrComplexType

types/reflection.go

@@ -393,6 +393,8 @@ func applyTag(structField *reflect.StructField, field *Field) error {
 			field.ValidChars = value
 		case "invalidChars":
 			field.InvalidChars = value
+		case "pointer":
+			field.Pointer = true
 		default:
 			return fmt.Errorf("invalid tag %s on field %s", key, structField.Name)
 		}

types/types.go

@@ -150,6 +150,7 @@ type Field struct {
 	Description  string      `json:"description,omitempty"`
 	CodeName     string      `json:"-"`
 	DynamicField bool        `json:"dynamicField,omitempty"`
+	Pointer      bool        `json:"pointer,omitempty"`
 }
 
 type Action struct {