We weren't appending the individual changes to the slice, but the entire slice.
Since "Change" is an "any", this isn't a type error. So make a Change a
non-empty interface (I had seen an issue like this coming, should have made it
an interface then, at least now we have a reasonable method, to get the modseq
of a change).
Found while working on an imap webpush prototype.
eadbda027c