json: use strconv.Append variants to avoid allocations in encoding
json: use strconv.Append variants to avoid allocations in encoding

Before/after, best of 3:
json.BenchmarkCodeEncoder  10  183495300 ns/op  10.58 MB/s
->
json.BenchmarkCodeEncoder  10  133025100 ns/op  14.59 MB/s

But don't get too excited about this.  These benchmarks, while
stable at any point of time, fluctuate wildly with any line of
code added or removed anywhere in the path due to stack splitting
issues.

It's currently much faster, though, and this is the API that
doesn't allocate so should always be faster in theory.

R=golang-dev, dsymonds, rsc, r, gri
CC=golang-dev
http://codereview.appspot.com/5411052

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