zstd: Remove offset from bitReader

[greatroar]

We can reslice instead of keeping a separate offset. This gets rid of some bounds checks. Also some other micro-optimizations to bit reading code. Combined results:

                                                     │   zstd/old   │              zstd/new               │
                                                     │     B/s      │     B/s       vs base               │
Decoder_DecoderSmall/kppkn.gtb.zst/buffered-8          427.6Mi ± 0%   428.2Mi ± 0%  +0.13% (p=0.019 n=10)
Decoder_DecoderSmall/kppkn.gtb.zst/unbuffered-8        511.6Mi ± 3%   516.9Mi ± 3%       ~ (p=0.280 n=10)
Decoder_DecoderSmall/geo.protodata.zst/buffered-8      1.110Gi ± 0%   1.110Gi ± 0%       ~ (p=0.165 n=10)
Decoder_DecoderSmall/geo.protodata.zst/unbuffered-8    824.7Mi ± 2%   827.3Mi ± 2%       ~ (p=0.481 n=10)
Decoder_DecoderSmall/plrabn12.txt.zst/buffered-8       330.4Mi ± 0%   330.3Mi ± 1%       ~ (p=0.645 n=10)
Decoder_DecoderSmall/plrabn12.txt.zst/unbuffered-8     533.3Mi ± 4%   538.8Mi ± 5%       ~ (p=0.393 n=10)
Decoder_DecoderSmall/lcet10.txt.zst/buffered-8         395.0Mi ± 0%   394.6Mi ± 0%  -0.10% (p=0.034 n=10)
Decoder_DecoderSmall/lcet10.txt.zst/unbuffered-8       556.5Mi ± 6%   546.2Mi ± 8%       ~ (p=0.436 n=10)
Decoder_DecoderSmall/asyoulik.txt.zst/buffered-8       342.2Mi ± 0%   342.2Mi ± 0%       ~ (p=0.956 n=10)
Decoder_DecoderSmall/asyoulik.txt.zst/unbuffered-8     436.7Mi ± 2%   435.4Mi ± 3%       ~ (p=0.739 n=10)
Decoder_DecoderSmall/alice29.txt.zst/buffered-8        335.6Mi ± 2%   337.0Mi ± 0%  +0.43% (p=0.000 n=10)
Decoder_DecoderSmall/alice29.txt.zst/unbuffered-8      552.6Mi ± 3%   550.7Mi ± 4%       ~ (p=1.000 n=10)
Decoder_DecoderSmall/html_x_4.zst/buffered-8           2.264Gi ± 0%   2.271Gi ± 0%  +0.29% (p=0.035 n=10)
Decoder_DecoderSmall/html_x_4.zst/unbuffered-8         1.558Gi ± 4%   1.554Gi ± 3%       ~ (p=0.579 n=10)
Decoder_DecoderSmall/paper-100k.pdf.zst/buffered-8     3.554Gi ± 5%   3.610Gi ± 0%  +1.59% (p=0.000 n=10)
Decoder_DecoderSmall/paper-100k.pdf.zst/unbuffered-8   1.701Gi ± 8%   1.709Gi ± 5%       ~ (p=0.631 n=10)
Decoder_DecoderSmall/fireworks.jpeg.zst/buffered-8     7.891Gi ± 4%   8.070Gi ± 0%  +2.26% (p=0.000 n=10)
Decoder_DecoderSmall/fireworks.jpeg.zst/unbuffered-8   3.062Gi ± 4%   3.129Gi ± 2%  +2.16% (p=0.002 n=10)
Decoder_DecoderSmall/urls.10K.zst/buffered-8           525.4Mi ± 6%   553.8Mi ± 0%  +5.39% (p=0.000 n=10)
Decoder_DecoderSmall/urls.10K.zst/unbuffered-8         763.7Mi ± 6%   819.7Mi ± 2%  +7.34% (p=0.000 n=10)
Decoder_DecoderSmall/html.zst/buffered-8               894.8Mi ± 0%   898.8Mi ± 2%  +0.45% (p=0.043 n=10)
Decoder_DecoderSmall/html.zst/unbuffered-8             722.3Mi ± 2%   717.7Mi ± 2%       ~ (p=0.912 n=10)
Decoder_DecoderSmall/comp-data.bin.zst/buffered-8      386.6Mi ± 2%   390.4Mi ± 0%  +1.00% (p=0.000 n=10)
Decoder_DecoderSmall/comp-data.bin.zst/unbuffered-8    145.2Mi ± 2%   148.7Mi ± 1%  +2.42% (p=0.003 n=10)
geomean                                                770.3Mi        777.5Mi       +0.93%

[klauspost]

👍🏼 Looks good! Thanks!