Micro-tweak u8 function

benchmark                      old ns/op    new ns/op    delta
BenchmarkVerticalScaler2           58283        53843   -7.62%
BenchmarkVerticalScaler4           98905        92305   -6.67%
BenchmarkVerticalScaler6          146408       124307  -15.10%
BenchmarkVerticalScaler8          173109       170509   -1.50%
BenchmarkVerticalScaler10         205611       198011   -3.70%
BenchmarkVerticalScaler12         242813       236813   -2.47%
BenchmarkVerticalScalerN          359020       336819   -6.18%
BenchmarkHorizontalScaler2         67503        66263   -1.84%
BenchmarkHorizontalScaler4        103305       102005   -1.26%
BenchmarkHorizontalScaler6        140608       133507   -5.05%
BenchmarkHorizontalScaler8        172609       171309   -0.75%
BenchmarkHorizontalScaler10       203911       197311   -3.24%
BenchmarkHorizontalScaler12       241513       239313   -0.91%
BenchmarkHorizontalScalerN        378821       370621   -2.16%
Fix ImageBicubicIDown benchmark
Prepare for use with asm versions
Add plane dumper helper
Add go asm generator

It's not perfect, syntax is a bit confusing between plan9 and intel.
Yet it does allow generating go asm code and is vastly better than
nasm/yasm macros.
Add sse2 horizontal scaler for 2-taps

Almost 8 times faster :D

benchmark                     old ns/op    new ns/op    delta
BenchmarkHorizontalScaler2        66503         8525  -87.18%
Add sse2 horizontal scaler for 4-taps

benchmark                     old ns/op    new ns/op    delta
BenchmarkHorizontalScaler4       102905        10225  -90.06%
Add sse2 horizontal scaler for 8-taps

benchmark                      old ns/op    new ns/op    delta
BenchmarkHorizontalScaler6        134307        16150  -87.98%
BenchmarkHorizontalScaler8        166209        16160  -90.28%
Add sse2 horizontal scalers for 10, 12 and n-taps

benchmark                      old ns/op    new ns/op    delta
BenchmarkHorizontalScaler10       206611        35022  -83.05%
BenchmarkHorizontalScaler12       240713        41482  -82.77%
BenchmarkHorizontalScalerN        375821        48622  -87.06%
Always use asm versions when available

h8scale6 should not be used anymore, but still, it doesn't hurt to
fallback to n-taps asm version if someone access it directly