Pry method patcher

I want to be able to redefine methods from the pry-rescue gem. At the moment I "can" by importing and sub-classing `Pry::Commands::Edit::MethodPatcher`, but it's nicer to provide an actual API for this in `Pry::Method#redefine`.

I want to eventually expand the scope of this to include saving methods too (but it's quite different from the save-source branch).

Any thoughts?