Added support for structs and strings in unions.

(C++ only for now).
Also fixed vector of union support in the object API.

Bug: 36902939
Change-Id: I935f4cc2c303a4728e26c7916a8ec0adcd6f84cb
Tested: on Linux.

tests/union_vector/union_vector.fbs

@@ -1,22 +1,29 @@
-table MuLan {
+// Demonstrates the ability to have vectors of unions, and also to
+// store structs and strings in unions.
+
+table Attacker {
   sword_attack_damage: int;
 }
 
-table Rapunzel {
+struct Rapunzel {
   hair_length: int;
 }
 
-table Belle {
+struct BookReader {
   books_read: int;
 }
 
 union Character {
-  MuLan,
-  Rapunzel,
-  Belle,
+  MuLan: Attacker,  // Can have name be different from type.
+  Rapunzel,         // Or just both the same, as before.
+  Belle: BookReader,
+  BookFan: BookReader,
+  Other: string,
+  Unused: string
 }
 
 table Movie {
+  main_character: Character;
   characters: [Character];
 }