ui: optimize buffered write with no label
This was spotted while making fastannotate faster again after ditching its
own formatter. Since I'm going to inline _write() into ui.write(), I decided
to include this patch in this series.
Here, the cost of '(self.label(a, label) for a in args)' was significant
in hot loops.
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']