Memory usage suggestion : reduce unnecessary boxes

[mgravell]

Currently, if you deserialize a model with lots of, say, boolean values in it: you get a lot of boxed booleans on the heap, from multiple sources, but mostly:

• JsonTextReader.SetToken
• new JValue(object, [JTokenType])

Suggestion:

• create static readonly boxed objects for a range of likely primitive values
• use the shared boxed copies in preference to vanilla boxing
• avoid naïve and unnecessary new JValue(object) (side benefit: avoid some extra type testing)

This would significantly help (based on real testing with dotMemory etc) both the temporary gen-zero allocations of in-flight objects and the cost of retained JToken etc trees.

Happy to submit a PR if useful.