json: add struct tag option to wrap literals in strings

json: add struct tag option to wrap literals in strings

Since JavaScript doesn't have [u]int64 types, some JSON APIs
encode such types as strings to avoid losing precision.

This adds a new struct tag option ",string" to cause
fields to be wrapped in JSON strings on encoding
and unwrapped from strings when decoding.

R=rsc, gustavo
CC=golang-dev
https://golang.org/cl/4918051