Bringing the buffered-20Hz-output from mux to unrepl is a bit more than copy & paste 😞

@dominicm thanks for your comiseration

with mux there’s only one writer to flush, with unrepl, one writer is created by evaluation (and can outlive it) so many writers have to be flushed, we either have to rely on weakrefs or yet another proxy to mark as dirty freshly written-to writers