The previous type cache is quadratic in time in the situation where
new types are continually encountered. Now that it is possible to dynamically
create new types with the reflect package, this can cause json to
perform very poorly.

Switch to sync.Map which does well when the cache has hit steady state,
but also handles occasional updates in better than quadratic time.

benchmark                                     old ns/op      new ns/op     delta
BenchmarkTypeFieldsCache/MissTypes1-8         14817          16202         +9.35%
BenchmarkTypeFieldsCache/MissTypes10-8        70926          69144         -2.51%
BenchmarkTypeFieldsCache/MissTypes100-8       976467         208973        -78.60%
BenchmarkTypeFieldsCache/MissTypes1000-8      79520162       1750371       -97.80%
BenchmarkTypeFieldsCache/MissTypes10000-8     6873625837     16847806      -99.75%
BenchmarkTypeFieldsCache/HitTypes1000-8       7.51           8.80          +17.18%
BenchmarkTypeFieldsCache/HitTypes10000-8      7.58           8.68          +14.51%

The old implementation takes 12 minutes just to build a cache of size 1e5
due to the quadratic behavior. I did not bother benchmark sizes above that.

Change-Id: I5e6facc1eb8e1b80e5ca285e4dd2cc8815618dad
Reviewed-on: https://go-review.googlesource.com/76850
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>