More quotedPrintEncode speed-ups

Starting at a baseline of around 4600 ns/op:
 (1) Expanding "append(buf[:0], c)" into "buf[0] = c; nextOut = buf[:1]"
     shaved off about 1000 ns/op.
 (2) Using a [256]byte table lookup for isPrintable shaved off another
     600 ns/op
Shaving a little more than 1/3 of the time off on average, at least for
our test message.