Update return-list grammar for forward/move (#726)

Author: ntrel

source/parse.h

@@ -7459,8 +7459,11 @@ class parser
     //G
     //G return-list:
     //G     expression-statement
-    //G     '->' type-id
-    //G     '->' parameter_declaration_list
+    //G     '->' return-direction? type-id
+    //G     '->' parameter-declaration-list
+    //G
+    //G return-direction: one of
+    //G     'forward' 'move'
     //G
     //G contract-seq:
     //G     contract