Feature request: "convert function to closure" assist

[jplatte]

It would be nice if I could easily turn a function that is declared inside another function / method into a closure (to enable further refactorings like removing a parameter because it can be captured instead).

[mdx97]

@rustbot claim

[mdx97]

Got something working @jplatte, need to write some tests and hoping to have a PR out tomorrow if I have the time :)