New rule FavourNestedFunctions

[knocte]

Testcases:

let Foo () =
    ()

let Bar () =
    ()

=> No errors

let private Foo () =
    ()

let Bar () =
    ()

=> No errors

let private Foo () =
    ()

let Bar () =
    Foo()
    ()

=> Error! (because Foo can be transformed from private to nested)

let Bar () =
    let Foo() =
        ()

    Foo()
    ()

=> no error (because Foo is nested)

let private Foo () =
    ()

let Bar () =
    Foo()
    ()

let Baz () =
    Foo ()
    ()

=> no error (because Foo is called by 2 functions: Bar and Baz; therefore, it cannot become nested)