Skip zeroing stack and add a bigger reusable buffer:

```
benchmark                                            old ns/op     new ns/op     delta
BenchmarkDecompress4XNoTable/digits-32               200409        151974        -24.17%
BenchmarkDecompress4XNoTable/gettysburg-32           2610          2565          -1.72%
BenchmarkDecompress4XNoTable/twain-32                558870        532480        -4.72%
BenchmarkDecompress4XNoTable/low-ent.10k-32          57291         53948         -5.84%
BenchmarkDecompress4XNoTable/superlow-ent-10k-32     15556         14443         -7.15%
BenchmarkDecompress4XNoTable/case1-32                296           257           -13.28%
BenchmarkDecompress4XNoTable/case2-32                250           206           -17.27%
BenchmarkDecompress4XNoTable/case3-32                257           217           -15.46%
BenchmarkDecompress4XNoTable/pngdata.001-32          75101         73473         -2.17%
BenchmarkDecompress4XNoTable/normcount2-32           414           319           -22.89%
BenchmarkDecompress4XNoTableTableLog8/digits-32      200054        153302        -23.37%
BenchmarkDecompress4XTable/digits-32                 200226        152865        -23.65%
BenchmarkDecompress4XTable/gettysburg-32             3815          3908          +2.44%
BenchmarkDecompress4XTable/twain-32                  557677        533928        -4.26%
BenchmarkDecompress4XTable/low-ent.10k-32            57732         54737         -5.19%
BenchmarkDecompress4XTable/superlow-ent-10k-32       16101         14918         -7.35%
BenchmarkDecompress4XTable/case1-32                  2035          2003          -1.57%
BenchmarkDecompress4XTable/case2-32                  2023          1962          -3.02%
BenchmarkDecompress4XTable/case3-32                  2036          1986          -2.46%
BenchmarkDecompress4XTable/pngdata.001-32            78191         76109         -2.66%
BenchmarkDecompress4XTable/normcount2-32             1454          1371          -5.71%

benchmark                                            old MB/s     new MB/s     speedup
BenchmarkDecompress4XNoTable/digits-32               498.99       658.03       1.32x
BenchmarkDecompress4XNoTable/gettysburg-32           593.10       603.62       1.02x
BenchmarkDecompress4XNoTable/twain-32                469.06       492.31       1.05x
BenchmarkDecompress4XNoTable/low-ent.10k-32          698.18       741.46       1.06x
BenchmarkDecompress4XNoTable/superlow-ent-10k-32     674.99       726.98       1.08x
BenchmarkDecompress4XNoTable/case1-32                185.82       214.27       1.15x
BenchmarkDecompress4XNoTable/case2-32                180.33       218.02       1.21x
BenchmarkDecompress4XNoTable/case3-32                186.94       221.15       1.18x
BenchmarkDecompress4XNoTable/pngdata.001-32          681.75       696.85       1.02x
BenchmarkDecompress4XNoTable/normcount2-32           210.23       272.66       1.30x
BenchmarkDecompress4XNoTableTableLog8/digits-32      499.88       652.33       1.30x
BenchmarkDecompress4XTable/digits-32                 499.45       654.19       1.31x
BenchmarkDecompress4XTable/gettysburg-32             405.81       396.10       0.98x
BenchmarkDecompress4XTable/twain-32                  470.06       490.97       1.04x
BenchmarkDecompress4XTable/low-ent.10k-32            692.86       730.77       1.05x
BenchmarkDecompress4XTable/superlow-ent-10k-32       652.15       703.85       1.08x
BenchmarkDecompress4XTable/case1-32                  27.02        27.46        1.02x
BenchmarkDecompress4XTable/case2-32                  22.24        22.94        1.03x
BenchmarkDecompress4XTable/case3-32                  23.58        24.17        1.03x
BenchmarkDecompress4XTable/pngdata.001-32            654.81       672.72       1.03x
BenchmarkDecompress4XTable/normcount2-32             59.83        63.47        1.06x
```