Revise should possibly do function deletion and re-insertion in a fixed world age.

[oscardssmith]

This might not be technically feasible, but as someone who from time to time messes around with the implimenation of push!(::Vector, x), I have several times run into the issue where Revise doesn't work because it dutifully removes the old method for push! and then is unable to add a new method because doing so requires push!ing to the methodtable. I acknowledge that this is somewhat pathological, but it would be great if it was fixable.