reflect: make Value an opaque struct

Making Value opaque means we can drop the interface kludges
in favor of a significantly simpler and faster representation.
v.Kind() will be a prime candidate for inlining too.

On a Thinkpad X201s using -benchtime 10:

benchmark                           old ns/op    new ns/op    delta
json.BenchmarkCodeEncoder           284391780    157415960  -44.65%
json.BenchmarkCodeMarshal           286979140    158992020  -44.60%
json.BenchmarkCodeDecoder           717175800    388288220  -45.86%
json.BenchmarkCodeUnmarshal         734470500    404548520  -44.92%
json.BenchmarkCodeUnmarshalReuse    707172280    385258720  -45.52%
json.BenchmarkSkipValue              24630036     18557062  -24.66%

benchmark                            old MB/s     new MB/s  speedup
json.BenchmarkCodeEncoder                6.82        12.33    1.81x
json.BenchmarkCodeMarshal                6.76        12.20    1.80x
json.BenchmarkCodeDecoder                2.71         5.00    1.85x
json.BenchmarkCodeUnmarshal              2.64         4.80    1.82x
json.BenchmarkCodeUnmarshalReuse         2.74         5.04    1.84x
json.BenchmarkSkipValue                 77.92       103.42    1.33x

I cannot explain why BenchmarkSkipValue gets faster.
Maybe it is one of those code alignment things.

R=iant, r, gri, r
CC=golang-dev
https://golang.org/cl/5373101

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