Implement some kind of effective entr

[rikhuijzer]

It would be very convenient if there would be some kind of entr_gen(; M) which tracks the changed methods. If a method changes, then it will re-run the code. However, it should not run all unchanged methods.

[rikhuijzer]

This issue is waiting on timholy/Revise.jl#622.

[rikhuijzer]

This is less of an issue if one uses

f() = gen(["myfile.md"]; M)
entr(f, ["contents"], [M])

because this will update all the code inside contents/myfile.md as soon as anything changes in contents/ or M.