This partially undoes 8fd7e6d070c8, but preserves its semantics.
More importantly, it results in the data about each fix being
decentralised, which makes it easier for new fixes to be added,
and other gofix users to slot new fixes in.

It also adds some useful metadata that could be used in the future.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5306092