opt: optimize decode slice

[liuq19]

minimal slice init cap:

cpu: Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz
                                          │ /tmp/tmpeu1jie64.source.txt │     /tmp/tmp6yk_dc8l.target.txt      │
                                          │           sec/op            │    sec/op     vs base                │
UnmarshalConcrete/CanadaGeometry_Sonic-8                    1.159m ± 4%   1.159m ± 10%        ~ (p=0.353 n=10)
UnmarshalConcrete/CitmCatalog_Sonic-8                       6.383m ± 3%   5.994m ±  5%   -6.10% (p=0.002 n=10)
UnmarshalConcrete/GolangSource_Sonic-8                      17.07m ± 2%   10.18m ±  4%  -40.38% (p=0.000 n=10)
UnmarshalConcrete/StringEscaped_Sonic-8                     93.61µ ± 4%   93.04µ ±  2%        ~ (p=0.436 n=10)
UnmarshalConcrete/StringUnicode_Sonic-8                     11.21µ ± 5%   10.95µ ±  2%        ~ (p=0.063 n=10)
UnmarshalConcrete/SyntheaFhir_Sonic-8                      11.552m ± 3%   9.464m ±  3%  -18.08% (p=0.000 n=10)
UnmarshalConcrete/TwitterStatus_Sonic-8                     2.537m ± 3%   2.287m ±  3%   -9.84% (p=0.000 n=10)
UnmarshalInterface/CanadaGeometry_Sonic-8                   3.427m ± 4%   3.342m ±  6%        ~ (p=0.075 n=10)
UnmarshalInterface/CitmCatalog_Sonic-8                      11.10m ± 3%   11.00m ±  3%        ~ (p=0.529 n=10)
UnmarshalInterface/GolangSource_Sonic-8                     19.36m ± 3%   19.44m ±  3%        ~ (p=0.684 n=10)
UnmarshalInterface/StringEscaped_Sonic-8                    106.2µ ± 4%   108.2µ ±  1%   +1.85% (p=0.023 n=10)
UnmarshalInterface/StringUnicode_Sonic-8                    26.58µ ± 6%   26.81µ ±  4%        ~ (p=0.971 n=10)
UnmarshalInterface/SyntheaFhir_Sonic-8                      13.72m ± 3%   13.44m ±  3%        ~ (p=0.143 n=10)
UnmarshalInterface/TwitterStatus_Sonic-8                    4.083m ± 2%   4.083m ±  2%        ~ (p=0.684 n=10)
geomean                                                     1.548m        1.448m         -6.47%

not allocate for empty slice:

[liuq19]

common benchmark in readme:

cpu: Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz
                                      │ /tmp/tmpwqjbs5pp.source.txt │     /tmp/tmp479rxvxm.target.txt     │
                                      │           sec/op            │   sec/op     vs base                │
Decoder_Generic_Sonic-8                                 290.3µ ± 3%   273.9µ ± 4%   -5.65% (p=0.000 n=10)
Decoder_Generic_Sonic_Fast-8                            215.0µ ± 2%   207.9µ ± 3%   -3.31% (p=0.009 n=10)
Decoder_Binding_Sonic-8                                113.65µ ± 4%   77.81µ ± 4%  -31.54% (p=0.000 n=10)
Decoder_Binding_Sonic_Fast-8                            86.51µ ± 5%   60.43µ ± 7%  -30.15% (p=0.000 n=10)
Decoder_Parallel_Generic_Sonic-8                        54.99µ ± 2%   54.42µ ± 4%        ~ (p=0.739 n=10)
Decoder_Parallel_Generic_Sonic_Fast-8                   44.81µ ± 4%   42.38µ ± 4%   -5.43% (p=0.001 n=10)
Decoder_Parallel_Binding_Sonic-8                        20.32µ ± 2%   12.92µ ± 3%  -36.44% (p=0.000 n=10)
Decoder_Parallel_Binding_Sonic_Fast-8                  17.542µ ± 4%   9.439µ ± 4%  -46.19% (p=0.000 n=10)
geomean                                                 69.41µ        54.27µ       -21.81%

                                      │ /tmp/tmpwqjbs5pp.source.txt │      /tmp/tmp479rxvxm.target.txt      │
                                      │             B/s             │      B/s       vs base                │
Decoder_Generic_Sonic-8                                42.82Mi ± 3%    45.39Mi ± 4%   +5.99% (p=0.000 n=10)
Decoder_Generic_Sonic_Fast-8                           57.82Mi ± 2%    59.79Mi ± 3%   +3.41% (p=0.009 n=10)
Decoder_Binding_Sonic-8                                109.4Mi ± 4%    159.8Mi ± 4%  +46.07% (p=0.000 n=10)
Decoder_Binding_Sonic_Fast-8                           143.7Mi ± 5%    205.8Mi ± 8%  +43.19% (p=0.000 n=10)
Decoder_Parallel_Generic_Sonic-8                       226.1Mi ± 2%    228.5Mi ± 4%        ~ (p=0.739 n=10)
Decoder_Parallel_Generic_Sonic_Fast-8                  277.5Mi ± 4%    293.4Mi ± 4%   +5.72% (p=0.001 n=10)
Decoder_Parallel_Binding_Sonic-8                       611.7Mi ± 2%    962.4Mi ± 3%  +57.34% (p=0.000 n=10)
Decoder_Parallel_Binding_Sonic_Fast-8                  708.7Mi ± 4%   1317.1Mi ± 4%  +85.85% (p=0.000 n=10)
geomean                                                179.1Mi         229.1Mi       +27.90%