Enable SingletonSupport by default

[k163377]

Currently kotlin-module creates a new instance by default when deserializing object.

This problem can be solved by enabling KotlinFeature.SingletonSupport.
On the other hand, to avoid compromising the integrity of Kotlin, this option should be enabled by default.

Barring any strong objections, I intend to make the above change in Jackson 3.0 (although it should be confirmed that there are no performance issues beforehand).
I would also like to remove the SingletonSupport option as well, as there seems to be no advantage to choosing this behavior.