x/tools/cmd: add tool to convert between functions and methods

[josharian]

Discussed with @alandonovan yesterday, recording here.

It would be useful to have a refactoring tool that could convert between:

func (t T) f(x int)

and

func f(t T, x int)

and clean up all the callers. This could be do with eg, but for common refactorings, it might be nicer to have a tool that can just do it, without having to write a template (or write a template generator), so that it can be invoked easily by IDEs. This probably just means new frontends to eg.