Use Vec + custom write logic instead of BTreeSet for NameRecords

[cmyr]

This will achieve the same thing (ensuring that we always sort before writing) but will have slightly better ergonomics.

We'll also need to make sure we dedup before writing.

We don't have mutable access to the objects when writing them. We could provide custom logic for writing that handled the sorting (there's an annotation for that) and given that we need a custom annotation anyway (to specify that we use BTreeSet, here) maybe that would be preferable? Shouldn't be too tricky...

Originally posted by @cmyr in #1162 (comment)