Reconcile the two versions of `adaptationfield.go`.

There are two files that deal with AdaptationFields:
- packet/adaptionfield.go: This one feels like the "right" one to use.
- packet/adaptationfield/adaptationfield.go: This one uses the older functional style.

I propose removing the `packet/adaptationfield/adaptationfield.go` file in favor of `packet/adaptationfield.go` as long as the functionality is the same and anything that is missing from `packet/adaptationfield.go` is filled in.