fix golint issues in core/lang (#492)

core/lang/lang.go

@@ -1,8 +1,11 @@
 package lang
 
+// Placeholder is a placeholder object that can be used globally.
 var Placeholder PlaceholderType
 
 type (
-	GenericType     = interface{}
+	// GenericType can be used to hold any type.
+	GenericType = interface{}
+	// PlaceholderType represents a placeholder type.
 	PlaceholderType = struct{}
 )