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This is not a complete JPEG implementation (e.g. it does not handle
progressive JPEGs or restart markers), but I was able to take a photo
with my phone, and view the resultant JPEG in pure Go.

The decoder is simple, but slow. The Huffman decoder in particular
should be easily improvable, but optimization is left to future
changelists. Being able to inline functions in the inner loop should
also help performance.

The output is not pixel-for-pixel identical to libjpeg, although
identical behavior isn't necessarily a goal, since JPEG is a lossy
codec. There are at least two reasons for the discrepancy.

First, the inverse DCT algorithm used is the same as Plan9's
src/cmd/jpg, which has different rounding errors from libjpeg's
default IDCT implementation. Note that libjpeg actually has three
different IDCT implementations: one floating point, and two fixed
point. Out of those four, Plan9's seemed the simplest to understand,
partly because it has no #ifdef's or C macros.

Second, fo...