optimize byte reading

[Stebalien]

When using cbor-gen, we usually work with byte buffers/readers. Calling ReadByte on a byte.Reader/byte.Buffer is much faster than calling Read and copying into a slice.

name            old time/op    new time/op    delta
Marshaling-4       911ns ± 2%     904ns ± 5%     ~     (p=0.646 n=9+10)
Unmarshaling-4    4.46µs ± 1%    3.94µs ± 2%  -11.55%  (p=0.000 n=10+8)
LinkScan-4        4.65µs ± 2%    4.01µs ± 2%  -13.76%  (p=0.000 n=9+10)
Deferred-4        3.68µs ± 3%    3.02µs ± 4%  -17.91%  (p=0.000 n=10+10)

name            old alloc/op   new alloc/op   delta
Marshaling-4        160B ± 0%      160B ± 0%     ~     (all equal)
Unmarshaling-4    3.46kB ± 0%    3.46kB ± 0%     ~     (all equal)
LinkScan-4          880B ± 0%      880B ± 0%     ~     (p=0.650 n=10+10)
Deferred-4         96.0B ± 0%     96.0B ± 0%     ~     (all equal)

name            old allocs/op  new allocs/op  delta
Marshaling-4        10.0 ± 0%      10.0 ± 0%     ~     (all equal)
Unmarshaling-4      43.0 ± 0%      43.0 ± 0%     ~     (all equal)
LinkScan-4          25.0 ± 0%      25.0 ± 0%     ~     (all equal)
Deferred-4          3.00 ± 0%      3.00 ± 0%     ~     (all equal)