Swtich from a sync.RWMutex to atomic.Value for cacheTypeFields.
On GOARCH=386, this recovers most of the remaining performance
difference from the 1.6 release. Compared with tip on linux/386:
name old time/op new time/op delta
CodeDecoder-40 92.8ms ± 1% 87.7ms ± 1% -5.50% (p=0.000 n=10+10)
name old speed new speed delta
CodeDecoder-40 20.9MB/s ± 1% 22.1MB/s ± 1% +5.83% (p=0.000 n=10+10)
With more time and care, I believe more of the JSON decoder's work
could be shifted so it is done before decoding, and independent of
the number of bytes processed. Maybe someone could explore that for
Go 1.8.