readLine is used in many text decoders, so it pays off if we care not to
do allocation for result on every readLine call:

    name      old time/op    new time/op    delta
    Speed-4      378ns ± 0%     379ns ± 1%     ~     (p=0.397 n=5+5)
    Decode-4    70.1µs ± 0%    67.3µs ± 0%   -3.97%  (p=0.008 n=5+5)
    Encode-4    16.6µs ± 0%    16.6µs ± 0%     ~     (p=0.548 n=5+5)

    name      old alloc/op   new alloc/op   delta
    Speed-4       280B ± 0%      280B ± 0%     ~     (all equal)
    Decode-4    32.3kB ± 0%    31.0kB ± 0%   -3.87%  (p=0.008 n=5+5)
    Encode-4    6.54kB ± 0%    6.54kB ± 0%     ~     (all equal)

    name      old allocs/op  new allocs/op  delta
    Speed-4       8.00 ± 0%      8.00 ± 0%     ~     (all equal)
    Decode-4       428 ± 0%       326 ± 0%  -23.83%  (p=0.008 n=5+5)
    Encode-4       297 ± 0%       297 ± 0%     ~     (all equal)