Hide singleton support

[dinomite]

Merge after #306

As discussed in issue #281, hide Kotlin singleton object deserialization behind a feature flag, enableExperimentalSingletonSupport.

[cowtowncoder]

I would actually suggest slightly different approach: use of an Enum, with descriptive names (and Javadocs matching). This because I think there are 3 options, as per earlier discussions:

1. Normal deserialize with no special handling (pre-2.10) -- possibly the default choice
2. Deserialize then canonicalize (i.e. deser but drop, use singleton): was default in 2.10
3. "Dummy" deserialize: skip content, return singleton. New choice that is probably better than (2) in that no alternate instances are created at all (but could hide issues wrt JSON structure?)

The main benefit, I think is just that there may be other approaches to add in future. Enum may also be used with annotations just in case there needs to be per-class override (granted for that may want programmatic access or something).

[dinomite]

Great idea, I'll get that updated shortly. Minor side benefit: one less boolean.

[dinomite]

Updated with an enum, let me know what you think. If this looks good, I'd like to merge this and leave the enhancements (dummy deserialize, those described in #281 etc.) to a separate PR.

[cowtowncoder]

Great, looks good. Apologies for delays; I will try to follow up more closely as this is the number 1 priority for me to get included before doing first (and maybe only) 2.11 pre-release candidate.

[dinomite]

👍 An RC for this sounds like a good idea to me.