Add functions for Nullable<_>

[gusty]

I think at least Option.ofNullableshould be added.

@mausch did a whole module some years ago, I don't think we should add all of them. Remember that Nullable are restricted to non value types, so things like Option.map would break.

[cmeeren]

Option.ofNullable has been part of FSharp.Core for some time now.

[gusty]

Thanks, for some reason I didn't realize it was already there. So I would say let's have a look at the other functions and if there's nothing interesting to add, that justify that module, we can close this issue.

[wallymathieu]

What about Nullable.ofOption for going from option to a Nullable<> type?

[cmeeren]

There's Option.toNullable built-in.

[wallymathieu]

Thanks @cmeeren