A refactor-enabling tool for go, in go.

This won't rewrite your files (yet). Since I write in Vim, I'd like to have a
more modular tool which will tell me where to change what. Then I can pipe this
through another tool (ed, awk, Vim, whatever).

We're starting with Rename Symbol. Hopefully, we'll have InlineVariable and InlineMethod
next, followed by ExtractVariable and ExtractMethod soon thereafter. Then,
IntroduceParameter and ChangeSignature.

That's probably 95% of what I use on a regular basis.