[#310] *: Implement string converters for enumerations

Implement `String` / `FromString` method pair in all levels of enum
definitions. From now `String()` returns canonical protojson-compatible
values.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/object/type.go

@@ -34,13 +34,36 @@ func TypeFromV2(t object.Type) Type {
 	}
 }
 
+// String returns string representation of Type.
+//
+// String mapping:
+//  * TypeTombstone: TOMBSTONE;
+//  * TypeStorageGroup: STORAGE_GROUP;
+//  * TypeRegular, default: REGULAR.
 func (t Type) String() string {
 	return t.ToV2().String()
 }
 
-// TypeFromString parses Type from its string representation.
-func TypeFromString(s string) Type {
-	return TypeFromV2(
-		object.TypeFromString(s),
-	)
+// FromString parses Type from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (t *Type) FromString(s string) bool {
+	var g object.Type
+
+	ok := g.FromString(s)
+
+	if ok {
+		*t = TypeFromV2(g)
+	}
+
+	return ok
+}
+
+// TypeFromString parses Type from its string representation.
+//
+// Deprecated: use FromString method.
+func TypeFromString(s string) (t Type) {
+	t.FromString(s)
+	return
 }