POC encoding/json.IndentWriter implementation

[flimzy]

This adds some Benchmarks for the existing Indent function, against the existing indent test inputs. A typical run:

BenchmarkIndentExample1-2       20000000               116 ns/op           8.56 MB/s          64 B/op          1 allocs/op
BenchmarkIndentExample2-2       10000000               194 ns/op          10.30 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample3-2       10000000               180 ns/op          11.10 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample4-2        5000000               323 ns/op          18.56 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample5-2       10000000               261 ns/op          11.49 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample6-2        5000000               365 ns/op          19.13 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample7-2        5000000               321 ns/op          21.76 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample7b-2       1000000              1004 ns/op         696.72 MB/s         880 B/op          8 allocs/op

Then it adds streaming support, and adds a new exported method, IndentWriter, which wraps an io.Writer, and indents the stream, as documented. Benchmark results virtually identical (within expected bounds for running tests on a VM), with the addition of a new set of tests for the streaming functionality:

BenchmarkIndentExample1-2               10000000               139 ns/op           7.19 MB/s          64 B/op          1 allocs/op
BenchmarkIndentExample2-2               10000000               189 ns/op          10.58 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample3-2               10000000               204 ns/op           9.79 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample4-2                5000000               330 ns/op          18.18 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample5-2                5000000               276 ns/op          10.84 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample6-2                3000000               409 ns/op          17.09 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample7-2                5000000               342 ns/op          20.42 MB/s          72 B/op          2 allocs/op
BenchmarkIndentExample7b-2               1000000              1165 ns/op         600.74 MB/s         880 B/op          8 allocs/op
BenchmarkIndentStreamExample1-2          5000000               323 ns/op           3.09 MB/s         152 B/op          3 allocs/op
BenchmarkIndentStreamExample2-2          5000000               367 ns/op           5.44 MB/s         160 B/op          4 allocs/op
BenchmarkIndentStreamExample3-2          5000000               363 ns/op           5.50 MB/s         160 B/op          4 allocs/op
BenchmarkIndentStreamExample4-2          3000000               496 ns/op          12.08 MB/s         160 B/op          4 allocs/op
BenchmarkIndentStreamExample5-2          3000000               455 ns/op           6.58 MB/s         160 B/op          4 allocs/op
BenchmarkIndentStreamExample6-2          3000000               640 ns/op          10.94 MB/s         160 B/op          4 allocs/op
BenchmarkIndentStreamExample7-2          3000000               520 ns/op          13.45 MB/s         160 B/op          4 allocs/op
BenchmarkIndentStreamExample7b-2         1000000              1293 ns/op         541.27 MB/s         960 B/op          9 allocs/op