Skip most of the scanning and parsing logic for simple (non-object/array) JSON values.

benchmark                   old ns/op    new ns/op    delta
BenchmarkUnmarshalInt             948          436  -54.01%
BenchmarkUnmarshalUint            930          427  -54.09%
BenchmarkUnmarshalString         1407          715  -49.18%
BenchmarkUnmarshalFloat          1114          536  -51.89%
BenchmarkUnmarshalBool            759          266  -64.95%
BenchmarkUnmarshalStruct         8165         8181   +0.20%

No significant effects on the go1 benchmarks:

benchmark                 old ns/op    new ns/op    delta
BenchmarkBinaryTree17    9647362752   9596196417   -0.53%
BenchmarkFannkuch11      5623613048   5518694872   -1.87%
BenchmarkGobDecode         32944041     33165434   +0.67%
BenchmarkGobEncode         21237482     21080554   -0.74%
BenchmarkGzip             750955920    749861980   -0.15%
BenchmarkGunzip           197369742    197886192   +0.26%
BenchmarkJSONEncode        79274091     78891137   -0.48%
BenchmarkJSONDecode       180257802    175280358   -2.76%
BenchmarkMandelbrot200      7396666      7388266   -0.11%
BenchmarkParse             11446460     11386550   -0.52%
BenchmarkRevcomp         1605152523   1599512029   -0.35%
BenchmarkTemplate         204538247    207765574   +1.58%

benchmark                  old MB/s     new MB/s  speedup
BenchmarkGobDecode            23.30        23.14    0.99x
BenchmarkGobEncode            36.14        36.41    1.01x
BenchmarkGzip                 25.84        25.88    1.00x
BenchmarkGunzip               98.32        98.06    1.00x
BenchmarkJSONEncode           24.48        24.60    1.00x
BenchmarkJSONDecode           10.76        11.07    1.03x
BenchmarkParse                 5.06         5.09    1.01x
BenchmarkRevcomp             158.34       158.90    1.00x
BenchmarkTemplate              9.49         9.34    0.98x

Fixes #3949.

R=golang-dev, dave, bradfitz, timo
CC=golang-dev
https://golang.org/cl/7068043