Added missing EnsureJs.
Removed JsBox and JsInstance.

JsBox was an intermediary step towards having fully untyped references, and this has been implemented. JsBox has gone and is replaced by object. As part of this migration, JsInstance, JsString, JsNumber and JsBoolean have also been removed, because they served no real purpose anymore.
Replaced strings in DefineProperty/DefineFunction with Id constants.
Removed Convert.To calls that could be replaced by casts.
Minimized the number of GetClass calls.