Support more `java.util` classes in default serializer

[jankotek]

Some well known JVM classes are not handled by Elsa. There are some problems with their deserializaiton, some fields are private, object stack can not be handled... etc

That includes:

• Calendar
• StringBuillder and StringBuffer
• Currency
• EnumSet
• TimeZone

Following classes are private, and there is problem with object stack on deserialization

• Collections.synchronized*()
• Collections.unmodifiable*()

Following singletons are registered in MapDB, but not in plain Elsa

• Collections.emptyList
• Collections.singleton
• Collections.emptyMap
• Collections.emptySet
• Collections.singletonList
• Collections.singletonMap

Elsa should also handle Externalizable from other frameworks, namely:

• KryoSerializable