We can't depend on init() order, and certainly we don't want to
register all future benchmarks that use jsonbytes or jsondata to init()
in json_test.go, so we use a more general solution: make generation of
jsonbytes and jsondata their own function so that the compiler will take
care of the order.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6282046