I'm not exactly sure why there's a performance gain, but it seems like
an easy win. Maybe it's a cache line thing. Maybe it's that
unsafe.Sizeof(decompressor{}) drops to below unmappedzero, so that
checkref/checkoffset don't need to insert TESTB instructions. Maybe
it's less noise for the conservative garbage collector. Maybe it's
something else.

compress/flate benchmarks:
BenchmarkDecodeDigitsSpeed1e4          378628       349906   -7.59%
BenchmarkDecodeDigitsSpeed1e5         3481976      3204898   -7.96%
BenchmarkDecodeDigitsSpeed1e6        34419500     31750660   -7.75%
BenchmarkDecodeDigitsDefault1e4        362317       335562   -7.38%
BenchmarkDecodeDigitsDefault1e5       3290032      3107624   -5.54%
BenchmarkDecodeDigitsDefault1e6      30542540     28937480   -5.26%
BenchmarkDecodeDigitsCompress1e4       362803       335158   -7.62%
BenchmarkDecodeDigitsCompress1e5      3294512      3114526   -5.46%
BenchmarkDecodeDigitsCompress1e6     30514940     28927090   -5.20%
BenchmarkDecodeTwainSpeed1e4           412818       389521   -5.64%
BenchmarkDecodeTwainSpeed1e5          3475780      3288908   -5.38%
BenchmarkDecodeTwainSpeed1e6         33629640     31931420   -5.05%
BenchmarkDecodeTwainDefault1e4         369736       348850   -5.65%
BenchmarkDecodeTwainDefault1e5        2861050      2721383   -4.88%
BenchmarkDecodeTwainDefault1e6       27120120     25862050   -4.64%
BenchmarkDecodeTwainCompress1e4        372057       350822   -5.71%
BenchmarkDecodeTwainCompress1e5       2855109      2718664   -4.78%
BenchmarkDecodeTwainCompress1e6      26987010     26336030   -2.41%

image/png benchmarks:
BenchmarkDecodeGray               1841839      1802251   -2.15%
BenchmarkDecodeNRGBAGradient      7115318      6933280   -2.56%
BenchmarkDecodeNRGBAOpaque        6135892      6013284   -2.00%
BenchmarkDecodePaletted           1153313      1114302   -3.38%
BenchmarkDecodeRGB                5619404      5511190   -1.93%

R=rsc, r
CC=golang-dev
https://golang.org/cl/6533048