Add implicit casts from a value to Option types

[declard]

Provides an implicit cast from of any type T to a corresponding Option of T

Useful in scenario like this:

Option<int> LovelyFunction()
{
    DoSomething();
    DoSomethingElse();
    return 42;
}

[nlkl]

Hi!

Thanks for your interest in Optional and this PR.

This has been considered previously, where we decided not to proceed: https://github.com/nlkl/Optional/issues/53

Once nullable reference types land, I might reconsider an implicit cast fromT? to Option<T>.

/ Nils