Improve marshal speed

[klauspost]

• Add fast case for non-escaped inputs.
• Add specialized float printing.

benchmark                                             old ns/op     new ns/op     delta
BenchmarkIter_MarshalJSONBuffer/apache_builds-32      287323        148202        -48.42%
BenchmarkIter_MarshalJSONBuffer/canada-32             13529136      10290900      -23.94%
BenchmarkIter_MarshalJSONBuffer/citm_catalog-32       1727759       1231152       -28.74%
BenchmarkIter_MarshalJSONBuffer/github_events-32      147846        72801         -50.76%
BenchmarkIter_MarshalJSONBuffer/gsoc-2018-32          7302709       3186714       -56.36%
BenchmarkIter_MarshalJSONBuffer/instruments-32        405617        249186        -38.57%
BenchmarkIter_MarshalJSONBuffer/numbers-32            1114819       803988        -27.88%
BenchmarkIter_MarshalJSONBuffer/marine_ik-32          15554551      13492569      -13.26%
BenchmarkIter_MarshalJSONBuffer/mesh-32               4519934       3556764       -21.31%
BenchmarkIter_MarshalJSONBuffer/mesh.pretty-32        4542054       3540605       -22.05%
BenchmarkIter_MarshalJSONBuffer/twitterescaped-32     1361729       653521        -52.01%
BenchmarkIter_MarshalJSONBuffer/twitter-32            1361365       635662        -53.31%
BenchmarkIter_MarshalJSONBuffer/random-32             1593711       927722        -41.79%
BenchmarkIter_MarshalJSONBuffer/update-center-32      1599886       778478        -51.34%

benchmark                                             old MB/s     new MB/s     speedup
BenchmarkIter_MarshalJSONBuffer/apache_builds-32      329.43       638.68       1.94x
BenchmarkIter_MarshalJSONBuffer/canada-32             154.50       203.11       1.31x
BenchmarkIter_MarshalJSONBuffer/citm_catalog-32       289.57       406.37       1.40x
BenchmarkIter_MarshalJSONBuffer/github_events-32      360.71       732.54       2.03x
BenchmarkIter_MarshalJSONBuffer/gsoc-2018-32          420.40       963.39       2.29x
BenchmarkIter_MarshalJSONBuffer/instruments-32        267.03       434.67       1.63x
BenchmarkIter_MarshalJSONBuffer/numbers-32            134.66       186.72       1.39x
BenchmarkIter_MarshalJSONBuffer/marine_ik-32          117.59       135.56       1.15x
BenchmarkIter_MarshalJSONBuffer/mesh-32               142.19       180.70       1.27x
BenchmarkIter_MarshalJSONBuffer/mesh.pretty-32        141.50       181.52       1.28x
BenchmarkIter_MarshalJSONBuffer/twitterescaped-32     342.88       714.45       2.08x
BenchmarkIter_MarshalJSONBuffer/twitter-32            342.97       734.52       2.14x
BenchmarkIter_MarshalJSONBuffer/random-32             289.55       497.42       1.72x
BenchmarkIter_MarshalJSONBuffer/update-center-32      333.12       684.62       2.06x