Tweaks related to value class support in deserialization

[k163377]

Fixed minor flaws discovered through the porting process to jackson-module-kotlin.

• Changed the name of ValueClassDeserializer to WrapsNullableValueClassDeserializer.
  • This is because it was thought to be misleading that it should be used for other than value class that wraps nullables.
• Changed to use the return value of the creator when initializing the deserializer of value class.
  • This is because there was no problem with this form as long as the target is handled by KotlinModule.
  • However, since unboxConverter is used by Instantiator, the initialization cost does not change much.