Equality on tree should work despite the fact that there are two constructors `Short` and `String`

[Yoric]

... otherwise, we can end up with two values that should be equal but are not.

[Yoric]

In my code, I hit the problem by calling json::from on a Vec<String>, which for some reason ended up converted as JsonValue::String, despite the fact that all the strings had length 1. My unit test, which followed a different code path, didn't agree.