Model object environment records as records

[bakkot]

For some reason, object Environment Records have two additional "fields" - the "binding object" and the "withEnvironment" - which are not stored as slots, just additional data. We should make them actually be fields, like we do for the additional data in function Environment records.

While we're at it, NewObjectEnvironment could take a boolean for withEnvironment, so that it could set up the whole record. That would be helpful for the HTML folks, which for some reason needs to make these things.