fix: implement StructLog.WriteTo WriterTo signature

[davidjsonn]

StructLog.WriteTo now matches io.WriterTo, counts written bytes, and returns the first write failure so go vet stops complaining. Streaming tracers notice those errors via l.Stop(err) and abort cleanly instead of soldiering on with a broken writer. The helper WriteTrace returns once a write fails for consistency. With these tweaks the package builds cleanly under go vet and holds up better when stdout/stderr is misbehaving.

[MariusVanDerWijden]

How are you using the structlogger? It looks a bit like a change for the sake of the change itself instead of a real functional necessary change imo