Rename second argument in Option.fold

[georgeci]

The name of the second argument "f" like a not very good solution. I like to use named argument for fold calling and any more functions. Of course, I can write:

 fold(isEmpty = {...}){value -> ...}

But I want to be able to write like this

 fold(isEmpty = {...},  some = {value -> ...})

[MarioAriasC]

I used f as convention on everything that is function; I could change it in a future release

[MarioAriasC]

fix on 1.2