Add an Fn function to help with type inference

[tautropfli]

I don't know if we already knew this but passing a method group to an identity function helps infer its natural type. This makes using our function and action extensions much more attractive.

To help with the readability I propose an alias for the Identity function called Fn.

before:

var pow = Curry(Math.Pow);
//        ^^^^^
// error CS0411: The type arguments for
// method 'Functional.Curry<...>' cannot
// be inferred from the usage.

after:

var pow = Curry(Fn(Math.Pow));
// or
var pow = Fn(Math.Pow).Curry();
// or with Apply:
var expTwo = Apply(Fn(Math.Pow), 2, __);

[FreeApophis]

Funny, I tried that before and I thought it didn't work.