The forwardCopy function could be re-written in asm, and the copyHuff
method could probably be rolled into huffmanBlock and copyHist, but
I'm leaving those changes for future CLs.

compress/flate benchmarks:
benchmark                                 old ns/op    new ns/op    delta
BenchmarkDecoderBestSpeed1K                  385327       435140  +12.93%
BenchmarkDecoderBestSpeed10K                1245190      1062112  -14.70%
BenchmarkDecoderBestSpeed100K               8512365      5833680  -31.47%
BenchmarkDecoderDefaultCompression1K         382225       421301  +10.22%
BenchmarkDecoderDefaultCompression10K        867950       613890  -29.27%
BenchmarkDecoderDefaultCompression100K      5658240      2466726  -56.40%
BenchmarkDecoderBestCompression1K            383760       421634   +9.87%
BenchmarkDecoderBestCompression10K           867743       614671  -29.16%
BenchmarkDecoderBestCompression100K         5660160      2464996  -56.45%

image/png benchmarks:
benchmark                       old ns/op    new ns/op    delta
BenchmarkDecodeGray               2540834      2389624   -5.95%
BenchmarkDecodeNRGBAGradient     10052700      9534565   -5.15%
BenchmarkDecodeNRGBAOpaque        8704710      8163430   -6.22%
BenchmarkDecodePaletted           1458779      1325017   -9.17%
BenchmarkDecodeRGB                7183606      6794668   -5.41%

Wall time for Denis Cheremisov's PNG-decoding program given in
https://groups.google.com/group/golang-nuts/browse_thread/thread/22aa8a05040fdd49
Before: 3.07s
After:  2.32s
Delta:  -24%

Before profile:
Total: 304 samples
         159  52.3%  52.3%      251  82.6% compress/flate.(*decompressor).huffmanBlock
          58  19.1%  71.4%       76  25.0% compress/flate.(*decompressor).huffSym
          32  10.5%  81.9%       32  10.5% hash/adler32.update
          16   5.3%  87.2%       22   7.2% bufio.(*Reader).ReadByte
          16   5.3%  92.4%       37  12.2% compress/flate.(*decompressor).moreBits
           7   2.3%  94.7%        7   2.3% hash/crc32.update
           7   2.3%  97.0%        7   2.3% runtime.memmove
           5   1.6%  98.7%        5   1.6% scanblock
           2   0.7%  99.3%        9   3.0% runtime.copy
           1   0.3%  99.7%        1   0.3% compress/flate.(*huffmanDecoder).init

After profile:
Total: 230 samples
          59  25.7%  25.7%       70  30.4% compress/flate.(*decompressor).huffSym
          45  19.6%  45.2%       45  19.6% hash/adler32.update
          35  15.2%  60.4%       35  15.2% compress/flate.forwardCopy
          20   8.7%  69.1%      151  65.7% compress/flate.(*decompressor).huffmanBlock
          16   7.0%  76.1%       24  10.4% compress/flate.(*decompressor).moreBits
          15   6.5%  82.6%       15   6.5% runtime.memmove
          11   4.8%  87.4%       50  21.7% compress/flate.(*decompressor).copyHist
           7   3.0%  90.4%        7   3.0% hash/crc32.update
           6   2.6%  93.0%        9   3.9% bufio.(*Reader).ReadByte
           4   1.7%  94.8%        4   1.7% runtime.slicearray

R=rsc, rogpeppe, dave
CC=golang-dev, krasin
https://golang.org/cl/6127064